Rebuild site on config changes

[mpvosseller]

If I run jekyll serve --watch the site does not seem to rebuild when a configuration file (_config.yml) changes.

Ideally --watch could watch config files too.

[parkr]

This is a larger structural change... right now we keep track of the site object and its configuration options are meant to be immutable within one process.

[parkr]

I guess ideally jekyll serve --watch would just re-run jekyll build and prop up a server. It'd have to fail when source or dest are changed, but only for serve. For build, it could just call jekyll build repeatedly.

[parkr]

But again, it's still a pretty hefty structural change. Hm.