Add slash command "/format" to automatically format PRs

[seisman]

Description of proposed changes

This PR adds a special github action to enable slash commands in comments. With the feature enabled, anyone, who have written permission to a pull request, can write /format at the first line of a comment to trigger the "format" action, which can format the codes automatically.

See #645 for context.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

[weiji14]

Ok, I've added an APP_ID and APP_PRIVATE_KEY secret to the PyGMT repo which generates a token using the GenericMappingTools bot account:

See if the suggested changes work, not sure if it will work on your personal branch though @seisman, since the APP is installed only on GenericMappingTools/pygmt.

[weiji14]

How about modifying the "Run make format and make check to make sure the code follows the style guide." line in https://github.com/GenericMappingTools/pygmt/blob/master/.github/PULL_REQUEST_TEMPLATE.md to mention the new /format command?

[seisman]

How about modifying the "Run make format and make check to make sure the code follows the style guide." line in https://github.com/GenericMappingTools/pygmt/blob/master/.github/PULL_REQUEST_TEMPLATE.md to mention the new /format command?

make check also runs flake8, and there is no way to format the codes following flake8 rules.

[seisman]

Perhaps we can add a note in the template like:

**Notes**

- You can write `/foramt` in the first line of a comment to automatically format your codes

[seisman]

Just made two more changes in 11b45b3 and 11b45b3

[weiji14]

Getting there!

[weiji14]

Awesome, thanks for the good work @seisman, can't wait to try this out properly!