Support used-defined keywords (in prettified code).

Often we want to highlight our own API in the code. Sometimes we want our DSL to stand-out. This patch makes this possible.

For example (in 'config.ld'):

    user_keywords = { 'import_from', 'T' }

By default these keywords use the CSS style of "globals". It's possible to style each one differently. For example, I use "T" for gettext. Since it's adjacent to strings (my code looks like `warning.text = T"You didn't provide a name"`) I want my T to have a string color:

    .keyword-T {
      color: #8080ff;
    }