Handle undefined excludedFiles

First of all thanks for the plugin! Integrating frontend Blanket.js in grunt is a godsend for automation.

## Excluded Files Bug

I'm not sure of an exact repro, but when I don't specify `options.excludedFiles` in a grunt file and get a coverage threshold failure, I get the grunt error:

```
Fatal error: Cannot call method 'indexOf' of undefined
```

This PR fixes that error with (I believe) the correct default behavior if no files are excluded.

## Other Things

*Separate Note*: I don't see an "issues" area to file specific tickets, so here are two other things that I noticed didn't work when trying to style check and test my change:

**Style Error**:
```
$ grunt jshint
Running "jshint:all" (jshint) task
Linting tasks/blanket_mocha.js...ERROR
[L393:C23] W004: 'failMsg' is already defined.
          var failMsg = stats.failures + '/' + stats.tests + ' tests failed (' +

Warning: Task "jshint:all" failed. Use --force to continue.

Aborted due to warnings.
```

**No Mocha test task**:

```
$ grunt test
Warning: Task "mocha" not found. Use --force to continue.

Aborted due to warnings.
```