Add tootctl emoji export

[lfuelling]

Hi,
this PR adds the tootctl emoji export command.
This command will export either all or only a specific category to a export.tar.gz file that can be imported using tootctl.

The usage (with all options) is as follows:

$ tootctl emoji export ~/foo --overwrite --shortcode --category bar

• ~/foo will result in ~/foo/export.tar.gz (path has to exist)
• -- overwrite will overwrite the export file if it exists
• --shortcode will use the configured shortcode as file name
  • ie. thonking.png instead of 9f2bf533eb0a7165.png
• --category <category> will only export a specific category. If no matching category can be found, all emoji will be exported.

Additionally a typo in the documentation of the import command was fixed.

[SuperSandro2000]

If the category I want to export does not exist I think it should error and tell me this instead of silently ignoring it so that I maybe miss a typo.

[SuperSandro2000]

Just some thoughts. Let me know what you think.

[lfuelling]

@SuperSandro2000 Thanks for the suggestions, I've updated the code accordingly :)