Configurable pygments linenos and cssclass (fixes #93)

We introduce the .frogrc parameters pygments-linenos? and
pygments-cssclass, which are passed to HtmlFormatter.  Other parameters
accepted by the latter, like anchoring prefixes, would be more useful if
the formatter's context were the whole post rather than individual
snippets, so i've opted for not adding further support for them.
Merge pull request #94 from jaor/lineno

Configurable pygments linenos and cssclass (fixes #93)
Bump to v0.20

And update README for commit aac194b
Add Racket 6.1 to test matrix.