Accept single filepath

This change allows you to run a single test file. I've added the `filePath` property to the config options as the documentation seems to suggest it would be the right place:

```
Options:
  --config, -c
      The path to a jest config file specifying how to find and execute tests.
```

Although I can see the merit in there being a `--file` command line flag too.

The purpose of this change is to make the Jest CLI easier to use with a grunt / gulp style work flow:

`Component.js` is changed -> grunt / gulp watch notices -> runs `spec/component-spec.js`

It looks like most of the code in this function is identical to some of the code within the `_runTestsOnPathPattern` function -- can we consolidate a bit?