fix. CodeIgniter/View/Parser::render allow other extension

[nyufeng]

Fix bug from #2086
reference CodeIgniter/View/View::render

[panigrc]

The extension should be configurable instead of hardcoded php.
I would expect a constant i.e. VIEW_FILE_EXTENSION
That's probably for another feature request sorry.

[jim-parry]

This should have unit tests to demonstrate that a .html or whatever view file can be successfully rendered.