make formats for configuration-block configurable

[dbu]

I love the tabbed configuration blocks from the symfony doc. i would love to reuse the same in the FOSHttpCache doc for varnish 3 vs varnish 4 configuration. there we use a literalinclude.

Any chance the configuration-block could be extended to support those two things? (configurable tab types and literalinclude)

[xabbuh]

see #20 for the configurable tab types

[xabbuh]

Using literalinclude does actually work. However, with the current implementation, the language to highlight MUST be specified explicitly. Is that sufficient or would it be better to derive it from the configured default language?

[dbu]

i can live with that. if it would fall back to the default but would be
possible to still include other fragments too and overwrite the language
explicitly, that would be fancy of course. but its totally good enough
like this.

[xabbuh]

#24 makes the language to be used with the literalinclude directive optional in configuration blocks.