Don't allow = in filenames unless quoted.

Relevant to #80, #139.
Add a test that var defs are not speculatively parsed as files.
Add a test confirming that you can use ) in shell expansions.

Addresses #118.
Merge branch 'develop' of https://github.com/Factual/drake into develop

* 'develop' of https://github.com/Factual/drake:
  Add a test confirming that you can use ) in shell expansions.
  Add a test that var defs are not speculatively parsed as files.
  Don't allow = in filenames unless quoted.