Fix respec warnings.

1. use CSS "highlight" class instead of obsolete "sh_javascript".
2. remove obsolete "extraCSS" property.
Add compositionStartOffset/compositionEndOffset as suggested in
Microsoft's proposal[1].
The text is copied from there.

[1] https://dvcs.w3.org/hg/ime-api/raw-file/default/proposals/IMEProposal.html
Changes updated.
Temporary: clone Overview.html as Overview2.html
test
Merge branch 'gh-pages' of github-w3c:tkochi-w3c/imeapi_draft into gh-pages