[Feature Request] User settings for customising the default message in operations.py

[ahegelund]

Story time:

As a user of this awesome feature I would like to be able to customise the default strings in the "create_with_description" operation which is currently hardcoded in operations.py. If I could do this (e.g. via the user settings file) I would be able to save myself a significant amount of time over the course of my usage by pre-setting a template which matches how I like to style my Trello card descriptions (specifically to be able to retain indent formatting in my Sublime Text notes and leverage Trello Markdown)

Example custom message:

[TITLE]

[`DESCRIPTION`]

Acceptance Criteria:

1. create_with_description message is set via the settings file
2. It is possible to edit the create_with_description message via the user settings file

Documentation:

• Trello markdown: https://help.trello.com/article/821-using-markdown-in-trello
• Specific lines in operations.py:

    def create_with_description(self, label=""):
        first  = "Replace this with the card name. The card will be saved when this tab is closed, leave this tab empty to cancel"
        middle = "Replace this with the card description, it can be multiline."
        last   = "You can create as many cards as you want, just split them by the card_delimiter (from the settings) and a newline." 
        message = "%s %s\n\n%s\n%s" % (label, first, middle, last)

[nicosantangelo]

Thank you so much for the detailed feature request. This seems fairly easy to implement and a cool feature!

As I said on #12 my free time is kind of lacking but I'll give it a go asap.
Once again, any PR is more than welcomed and I can publish the new version of the package.