Add ability for users to modify a config value with their preferred editor

[bpeterso2000]

Use the editor feature in click to allow users to modify a config value.

with Config('myapp') as cfg:
    cfg.data.some_value = cfg.edit('some default text')

Editor settings:

# get the editor
cfg.editor

# set the editor
cfg.editor = 'emacs'

# delete the editor; use auto-detection (this is the default) 
cfg.editor = None

[bpeterso2000]

This enhancement is no longer needed.