Better regex to detect inline HTML tags.

Provides support for inline HTML tag syntax:
http://rdoc.info/github/slim-template/slim#Inline_tags

while excluding regular use of the colon character within text.

Inline HTML tags may have attributes with values. Tags are colored
as one would expect and attribute value strings are colored as
strings.

Caveat: String coloring is a bit sketchy. Gets tag coloring when
it fails.