Add ability to switch tracklists by exact username

This commit adds user switching to the player, accessed by the u
key. The user ID is fetched using /resolve, and the tracks
are accessed using /tracks.

Additionally, this commit adds a clear() method to UI::View,
which is used to reset the tracks view when switching users.
Move getstr prompt from hardcoded to parameter
Remove readme inconsistency

Technically, four-tet is still hardcoded as the default, but
it is now possible to switch users, so this is a bit misleading
Merge branch 'tthieman/user_switch'

Conflicts:
	README.md
	lib/soundcloud2000/ui/input.rb