Support .envrc file

[spenserblack]

Description

.envrc are files used by direnv. direnv loads the file into a bash sub-shell, making this file effectively bash.

Taking some of the other Shell/filenames samples as an example, I figured a smaller sample would be OK. Possibly preferable, since a larger sample would likely repeat export a bunch. If a larger sample is preferable, this might work.

Checklist:

• I am adding a new extension filename to a language.
  • The new extension filename is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A.envrc+export
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/jenkinsci/mesos-plugin/blob/7da7bc60883eec7fbb8e1fe888500d735bd297ba/.envrc
    • Sample license(s):
      • https://github.com/jenkinsci/mesos-plugin/blob/7da7bc60883eec7fbb8e1fe888500d735bd297ba/LICENSE
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[lildude]

LGTM. Thanks.

Note: this PR will not be merged until close to when the next release is made. See here for more details.