Command for closing currently open tag.

Triggered with ‘cmd-alt-.’ the command finds if the cursor is in a
position where an html tag is open and not closed afterwards. If so it
inserts the corresponding closing tag at the insertion point.
Fixed spec.

One of the spec erroneously triggered ‘language-html:close-tag’ instead
of ‘bracket-matcher:close-tag’.
closeTag is now available only in html grammars.
Fixed bug.

Mistakenly unsubscribed the whole editorView instead of closeTag
command when the grammar changed.

Also, the spec did not wait for the language-html activation (causing
the correct grammar not to be set).
closeTag: improved grammar recognition.

The closeTag is enabled when the grammar matches a certain
pattern. Before this change the check was grammar.name == 'HTML',
now it suffices that its scope name matches /^text.html/.
Merge pull request #45 from boborbt/master

Command for closing current tag.