Configure linter to disallow mocha's .only feature

[dkniffin]

Right now, we have nothing to ensure that people don't forget to remove their .onlys. This eslint-mocha plugin config will allow us to test for that. We'll need to add this package to the list of dependencies, and configure the eslint configs. This issue is just the package part. The linter part is smashingboxes/eslint-config-smashing-boxes#10

In order to allow for using .only in local development, we'll also want to change this line to posttest.

cc @leeallen337