Add new option `fileHeader`

It would be awesome to make it possible to define a file header like the following example.

    option.fileHeader: '# Changelog'

This is accomplished: 

* Add example tasks using the `fileHeader ` option.
* Add tests.
* Update readme.

The code is a bit ugly because it needs to handle `options.insertType === 'prepend'`.
I could have moved the handling to the `case 'prepend':` ([L113](https://github.com/ericmatthys/grunt-changelog/pull/18/files#diff-3e81f8611acaed53f40d1a3e5934e6b7R113)) and get rid of that regexp. But that has its own ugliness because then my code would be cluttered all over `function writeChangelog()`.

So I guess ist better to have all code related to `option.fileHeader` in a single condition like:
````
if (options.fileHeader) { 
  // do stuff
}
````

Closes #14 
Change already described [here](https://github.com/ericmatthys/grunt-changelog/pull/16/files#diff-3e81f8611acaed53f40d1a3e5934e6b7L38).