Use fetch_member instead of get_member for auto-converting

[hpenney2]

Using get_member will return None if the bot doesn't have the server members intent or hasn't (fully) chunked the guild the member came from. fetch_member doesn't require intents, so it's a better choice. This also requires process_options to be async so it can call and get the return values of coroutines (in this case, just fetch_member). However, this shouldn't cause any issues.

[eunwoo1104]

The problem with fetch things is that if bot fails fetching it will raise exception instead of returning None. However getting from cache gives None if it failed finding a user. But I'll reconsider before closing or merging.

[eunwoo1104]

Merging to new branch, thank you for contributing!