Generalized support for code highlighting

[lbertge]

Summary

PR GH-690 introduces the addition of 'py' to help label python-highlighted code in a document. I extend this idea by creating a Stylus variable to represent all default labels, and can be easily changed via override.styl.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

Reason: Currently the default theme uses a list of labels (line 98 in code.styl) to determine which code blocks will include the label in the top right corner. I think moving this list into config.styl and assigning a variable (named $codeLang) is better for a couple of reasons:

1. Easy overriding by override.styl. I think everyone will have different use cases for Vuepress, so they may choose to use only certain code languages. For example, a front-end heavy documentation may use lots of js, ts, etc, whereas a back-end heavy documentation may use go, python, rust.
2. Choosing only certain languages reduces the css footprint by a small amount.

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[meteorlxy]

May not put this after // colors