Enable ng-include directive to take a method.

This allows a user to bypass the template lookup much in the same way
that a directive works. if you simply set `template:`

This ability comes in quite handy in the situation where your app is
using precompiled templates to reduce network requests.

Prior to this fix a user had several options to hack around this.

They could define template script tags with the id's matching foo.html
or they could pre-seed the template cache on app.run so that the network
request would not happen.

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If you guys are interested in this, I can add tests and docs in a snap.