Better support for 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. Before these commits, something like `p Lorem ipsum dolor: sit amet.` would result in `dolor: sit` acquiring tag coloring. Not anymore.

Also, inline HTML tags may have attributes with values and tags will remain colored as one would expect as do attribute value strings.

**Caveat:** String coloring is a bit sketchy. Strings get tag coloring in certain situations, which is better than it was before. Inline HTML tags in Slim aren’t practical for long lists of tags with a bunch attributes each, anyway. The last tag in the line (along with its attributes and values) will color reliably, so the common use case:
```
ul
  li.first: a href="#about" title="About Us" target="_top" About Us
  li: a href="#contact" title="Contact Us" target="_top" Contact Us
```
should work like a charm.

**EDIT:** Also, added a commit to define a pattern for markdown, so that nested markdown doesn’t get Slim coloring.