try to preserve other matrix variables

Two things I'm not sure about:

-Does it make sense to arbitrarily preserve the line number of other matrix variables? They might not make sense anymore.
-Can I actually do what I did in the map function?
With complex regular expressions, it's helpful to have a comment with an example of the type of input it's going to parse and also what the output will be.  So here, we cold have something like:
```
# Input: TEST_SUITE="spec/a.rb spec/b.rb" RUN_JS="true"
# Output: { 'TEST_SUITE' => 'spec/a.rb spec/b.rb', 'RUN_JS' => 'true' }
```