Build HTML documentation automatically during the build process

This is an improvement as requested in #322 to update the HTML docs
after geany.txt or geany.css have changed.
Also, with this change the resulting HTML will be generated in the outdir,
not in srcdir/doc anymore.
Update Waf to 1.7.16
Add "hackingdoc" as proper action and remove non-working --hackingdoc option
Add option --disable-html-docs
The missing bits for HTML docs: only on GIT build and support in-tree geany.html