Accept BLANKET_CONFIG env var for alternate config file path

[JamesMGreene]

A BLANKET_CONFIG file will contain ONLY the JSON that is normally
accessed from a 'package.json' file's 'config.blanket' object

For example, in "package.json":

{
  "config": {
    "blanket": {
      "pattern": "src"
    }
  }
}

Equivalent in a standalone config file, e.g. ".grunt/blanket.json":

{
  "pattern": "src"
}

Usage (in Bash):

BLANKET_CONFIG=.grunt/blanket.json mocha --require blanket -R html-cov > coverage.html

Purpose:

Having this configuration optionally stored in a separate file has [at least] the following benefits:

• Externals tools (e.g. Grunt, etc.) would be able to generate a config file on the fly without affecting the very important "package.json" file
• Provides a better separation of concerns for configuration files, just as JSHint, JSCS, Travis CI, etc. all use their own config files rather than piggybacking on "package.json"
• For Node modules, this de-bloats the "package.json" file a bit, so there are fewer bits published to NPM

[JamesMGreene]

Corresponding PR for travis-cov: alex-seville/travis-cov#10

[JamesMGreene]

@alex-seville: Any idea why the Travis CI build failed? If I run grunt && npm test locally, it succeeds just fine.

[JamesMGreene]

Ping @alex-seville

[alex-seville]

Not sure why it failed before, but now there are merge conflicts on the PR

[alex-seville]

If the merge conflict is fixed I think this would be great to merge. Sorry for the delay.