Added ability to specify HTML tags for custom replacements

[intellix]

This is in reference to the issue: https://github.com/callumlocke/grunt-cdnify/issues/3

I'm not quite aware how to attach pull-requests to an already created issue, so just linking them together like this.

[intellix]

Just found another use-case for this. I've got apple-touch-icons and can now add those as well:

{
    "html": {
        "img[src]": "src",
        "link[rel=stylesheet]": "href",
        "link[rel=apple-touch-icon]": "href",
        "script[src]": "src",
        "img[ng-src]": "ng-src"
    }
}

[callumlocke]

thanks, I modified this slightly before merging... ng-src images aren't in the default set, but you can declare it manually.

check the readme and let me know if anything is unclear.

[intellix]

ah yes, I probably should have done that also. Thanks :)