Fetch roles using a converter

[Gehock]

Will fix #54 and #55. Blocks #6

Adds a new converter ArgRole that allows referring to roles as as follows:

• Additional role emoji (e.g. :one:)
• Additional role numeral (e.g. one)
• Additional role number (e.g. 1)
• Any role emoji (e.g. :ZEUS:)
• Any role name (e.g. Zeus)

Additionally, changes some of the existing commands to use the converter instead of parsing role names manually.

[Gehock]

This turned out to be slightly more complicated than anticipated because parsing the role requires knowledge about the Event argument as well. I'm not sure if the converter can access other arguments passed to the command

[Gehock]

Figured out that the converter can access the Event argument through ctx.args. This seems to be working now, could maybe be tested by implementing #80