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 mirrors textmate/bundle-support.tmbundle@0a12001c335da1e739a5a6d64368bb781914f50f
Add new typing pairs from the text bundle
Merge branch 'master' of https://github.com/textmate/markdown.tmbundle

# By Allan Odgaard
# Via Allan Odgaard
* 'master' of https://github.com/textmate/markdown.tmbundle:
  Add new typing pairs from the text bundle
  Allow markdown parameters to be set via TM_MARKDOWN