Move g:skeleton_template_dir into autoload/
Factor out skeleton#EditTemplate()
Rework custom placeholder API

Custom placeholders are now defined as anonymous functions in a dict,
g:skeleton_replacements or g:skeleton_replacements_{filetype}.
Document g:skeleton_replacements_{filetype} dicts
Restore About section to docs
Add Features section to README
Version 0.2.0

* Rework custom placeholder API.
* Add documentation with vimdoc.
* Add addon-info.json (VAM info file).