Only apply Markdown emphasis when there is an adjacent space

[krono]

Currently, it feels like the markdown display will simply jam markdown interpretation of * and _ on any matching pair.
(very simplified description)

This upsets quite some usages.

• the * is the most widely adopted (and prefered by affected people) way to indicate inclusive gender designations in German. This is messed up on display when people use the * that way, by, for example, saying Entwickler*innen und Tester*innen (the * disappear and the center part will be italic). See issue Gendersternchen incorrectly parsed as Markdown italics #1290.
• Simple multiplication is messed up in a similar way, see Excessive markdown parsing #1509. Most importatnly, this is the use the * was initially intended for.
• Bulleted lists. Markdown bold/italic handling confused by bullet lists #1226
• Workaround proposals exist (Unwanted markdown interpretation in posts #1877, Add a per-post markdown toggle when viewing posts #1402) but are pretty elk-specific. (ie, posts by non-elk-users will still look funky)

The same holds for _, although to a lesser extent.
_ has its uses in IT-related posts, and not everyone wants to use backquotes (because, again, they might not be using elk or any markdowning client in the first place.

Most "funnily", C function signatures are all over the place. Consider:

int	 rename (const char *__old, const char *__new);
extern int fseeko64 (FILE *__stream, __off64_t __off, int __whence);

Proposal

---

Only apply emphasis to text iff:

1. text starts with any space
2. followed by one to three of * or _ (adjacent space to the left)
3. followed by at least one character not being * or _. Maybe has * or _ embedded ¹
4. followed by one to three of * or _, that match the characters found in 2., but mirrored.
5. optionally followed by puctuation (eg, .,;?:!)
6. followed by at least one space (adjacent space to the right).

---

Test cases

This *has* emphasis.
This th*ng does n*ot.
This one is *debat*able*

from markdownguide

I just love **bold text**.
I just love __bold text__. 
NOT: Love**is**bold -- see caveat
Italicized text is the *cat's meow*.
Italicized text is the _cat's meow_.
NOT: A*cat*meow -- see caveat
This text is ***really important***.
This text is ___really important___.
This text is __*really important*__.
This text is **_really important_**.
NOT: This is really***very***important text. -- see caveat

Commonmark

YES: *foo bar*
NO: a * foo bar*
NO: a*"foo"*
NO: * a *
NO: *$*alpha.
NO: *£*bravo.
NO: *€*charlie.
YES: foo*bar* --- debatable
NO: 5*6*78 -- see caveat

NO: _foo*
NO: *foo bar *
NO: 
*foo bar
*
NO:  *(*foo)
YES: *(*foo*)* -- debatable, not considered here
YES: 
*foo
bar*
YES: *foo **bar** baz*
NO: *foo**bar**baz* -- see caveat

(_, double-*, and double-_ cases ommitted, but its virtually identical)

Caveat

This on purpose violates the recommendation in this markdown guide and commonmark.
The guide, for example, recommends rendering A*cat*meow with inter-word-emphasis, which is explicitly not recommended here.
Commonmark already restricts it to inner-word-*, disallowing _:

internal emphasis: foo*bar*baz
no emphasis: foo_bar_baz

The case foo*bar* from commonmark is debatable. applyinh

Footnotes

1. This restriction is only a simple case. Naurally, nesting, or switching between italic/bold emphasis needs more work. Also, singular, non-matched * might be allowed. ↩