Allow custom test functions

[alexlafroscia]

This feature is useful if using this plugin with a package like ember-mocha, which provides its own set of describe functions. A custom array can be provided so that those extensions can also be protected from having only called.

One thought about my changes is that it introduces a configuration object rather than an array. We could make the option parameter just an array, but I kind of like the extra context provided by the named keys. The difference is basically between this (how I have it right now)

{
     "rules": {
         "mocha/no-exclusive-tests": ["error", {
             "additionalTestFunctions": [
                 "describeModule"
             ]
         }]
     }
 }

and this (which I think provides less context and it more confusing)

{
     "rules": {
         "mocha/no-exclusive-tests": ["error", [
             "describeModule"
         ]]
     }
 }

Closes #80

[coveralls]

Coverage remained the same at 100.0% when pulling 1026819 on alexlafroscia:allow-custom-test-functions into 6594359 on lo1tuma:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 8c28af6 on alexlafroscia:allow-custom-test-functions into 6594359 on lo1tuma:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 8c28af6 on alexlafroscia:allow-custom-test-functions into 6594359 on lo1tuma:master.

[coveralls]

Coverage remained the same at 100.0% when pulling a2cbf40 on alexlafroscia:allow-custom-test-functions into 6594359 on lo1tuma:master.

[alexlafroscia]

Sorry about the messy Coveralls comments, I overwrote my commits a few times.

[lo1tuma]

Thanks for the Pull Request.

It seems like adding those options to each rule introduces a lot of duplication, especially if we add more rules to this plugin in the future.
What do you think about using shared settings (e.g. as eslint-plugin-import or eslint-plugin-backbone does)?
With shared settings the configuration would look like this:

{
     "rules": {
         "mocha/no-exclusive-tests": "error",
         "mocha/no-skipped-tests": "error"
     },
    "settings": {
        "mocha/additionalTestFunctions": [ "describeModule" ],
        "mocha/additionalXFunctions": [ "xcustom" ]
    }
 }

or

{
     "rules": {
         "mocha/no-exclusive-tests": "error",
         "mocha/no-skipped-tests": "error"
     },
    "settings": {
        "mocha": {
            "additionalTestFunctions": [ "describeModule" ],
            "additionalXFunctions": [ "xcustom" ]
        }
    }
 }

[alexlafroscia]

Huh, that's really cool. I didn't know you could configure ESLint that way -- I like it much better. Totally agree that the current approach means duplication, I'll fix up this PR to implement with shared settings instead.

[alexlafroscia]

Added the shared config just like @lo1tuma suggested, allowing either the one-line or nested option object.

[lo1tuma]

Great work @alexlafroscia.

Thank you 🍻 .