[PATCH 0 of 8] extensions: overhaul on descriptions

[Cédric Duval]

This patch series improves the handling of extensions help texts,
in particular:

 - patches 1 to 5: refactoring, and paving the way for the following
   patches, progressively moving code into a new module extdoc used
   both by hg and external tools

 - patches 6 and 7: support for a pre-processed list of extensions, as a
   fallback in case descriptions can't be fetched at runtime (issue1708)

 - patch 8: new generated section in the hg.1 presenting each extension

Thanks to Martin for the preliminary review.
-- 
Cédric