Allow markdown parameters to be set via TM_MARKDOWN

For example you may want to set:

    TM_MARKDOWN = "/path/to/multimarkdown --nosmart"

Previously this would fail because TM_MARKDOWN was treated as a single command (with possible spaces in its path).

This commit makes the same change for TM_SMARTYPANTS.
Change html_header() to call ruby18 instead of TM_RUBY

Bundle support ruby codes are currently have not been updated for ruby 2.0,
so anything using bundle support ruby codes they must be run on ruby 1.8.
Merge remote-tracking branch 'textmate/master'
Merge branch 'master' of github.com:prymatex/bundle-support.tmbundle