Allow users to customize the Markdown highlighter code

[hegemonic]

For code snippets, jsdoc/util/markdown applies a specific set of classes to the HTML tags. This approach only works if the template is using Google Code Prettify for syntax highlighting. We should allow users to override the built-in highlighter function so that it's possible to use other syntax highlighters and highlight code at the time the docs are generated.

There are a few ways we could implement this feature:

• Allow the user to specify a highlight function in a JS config file. This forces people to use a JS config file (but maybe that's not a bad thing).
• Allow the user to specify the location of a module that exports a highlighter function. This would be more cumbersome than the previous approach, but it would work with JSON config files. We could do this in conjunction with the previous approach.
• Allow the user to specify a format string for the code wrapper. This is probably not flexible enough.
• Allow the template to export a highlight function. This would require JSDoc to load the template before it could do anything Markdown-related, so this is a non-starter.

[hegemonic]

Fixed on master. Users can now do either of the following:

• Assign a highlighter function to markdown.highlight in a JS config file.
• Specify the path to a highlighter module in markdown.highlight in a config file. The highlighter module should export a method called highlight. (This module could be the publish.js file for a JSDoc template.)

Either way, the highlighter function should look like function highlight(code, language) {}, where code is the contents of the code block and language is the language the user specified, if any. The highlighter function should escape the code block's contents and wrap them in <pre><code> tags.