Add "extras" library that is imported to napari document

[tlancon]

This extra .py file can be a place for the user to dump extra functionality that will be imported into napari when the overlay script is opened.

For example

[tlancon]

Shortcut functions:

• reset_gamma(ignore=[]) resets the gamma value of all layers to 1, ignoring the index of any layers provided in the "ignore" list
• reset_luts(ignore=[]) resets all LUT-related parameters to their defaults, ignoring the index of any layers provided in the "ignore" list
  • This is contingent on running something like "get_defaults" on opening to remember all of the default parameters

Could potentially add a dock widget with buttons to perform each of these actions