Use unist trees for all Markdown values

[jfirebaugh]

The "comment" object (documentationjs/documentation-schema#1 / #18) should have no fields whose type is "Markdown-formatted string". Instead, the type of such fields should always be "unist tree", with all markdown syntax and inline jsdoc @links and such parsed into an AST.

Downstream modules such as themes should never need to require('jsdoc-inline-lex') or use remark's process. All they should do is traverse an AST, generating output.