new ac-source `ac-source-html-attribute-value`

read files for completion:
- html-stuff/html-attributes-complete/<TAG>-<ATTRIBUTE>
- html-stuff/html-attributes-complete/global-<ATTRIBUTE>
populate html-stuff/html-attributes-complete
ac-html merge
add some customization hint to readme
remove [\d+]
minor
Merge pull request #13 from osv/attr_complete

Attr complete
Update README.md