Allow plugins to hook into File > Open

[jitseniesen]

This is inspired by spyder-ide/spyder-notebook#134. Users of the spyder-notebook plugin expect to open a notebook using File > Open. At the moment, this causes the notebook to be opened in the editor, which is not so useful. Somehow the notebook plugin (or any other plugin) should be able to indicate that it should be used to open files of a particular type.

Maybe the plugin initialization code should include a call like:

register_file_extension('.ipynb')

When the user opens a file with that extension, Spyder calls the plugin's load() function, similar to Editor.load().

[ccordoba12]

Some comments about this:

1. I agree with the register_file_extension name.
2. This needs to go to to the spyder.api module.
3. You need to modify the method called open_file from app/mainwindow.py, present here (for now).