Add support for {...} specifier

[hollasch]

The pattern a{c,g,l,p,r,t}e should match 'ace', 'age', 'ale', 'ape', 'are', 'ate'.

Stretch one would be {,x,y} to match '', 'x', and 'y'.

Stretch two would be to match {a?a,b?b,c?c}.

[hollasch]

Note that this should also allow a user to specify multiple roots, like so:

pathmatch {D:\scratch;C:\scratch;%UserProfile%\scratch}/.../foo.cmd

Options (need a term for arguments inside braces) at the beginning of a pattern are a special case, and should be optimized as such.

In addition, consider implicit support for embedded spaces. That is, should we implicitly concatenate arguments when we have not encountered an expected closing brace? Inclined to think not.