Add override for the application ID in order to support legacy applications

[DXsmiley]

About this pull request

Allows users to manually specify the application ID, rather than having the library attempt to derive it from the bot's user ID. This is important for legacy bots where the application ID and bot user ID are not the same.

Changes

Adds an application_id argument to SlashCommand in order to manually specify the application ID. Changes are non-breaking.

Checklist

• I've checked this pull request runs on Python 3.6.X.
• This fixes something in Issues.
  • Issue: Lib does not account for legacy bots/apps #108, Incorrect assumption that bot user ID is the same as the application client ID #112
• This adds something new.
• There is/are breaking change(s).
• (If required) Relavent documentation has been updated/added.
• This is not a code change. (README, docs, etc.)

[eunwoo1104]

Oh, I forgot about this method. Anyways, merging this pull request, thank you for contributing!

[Kowlin]

Thank you so much for this PR! Was gonna do it my self at some point, but didn't find the time.