allow user to add gamepad profiles without tempering w/ core

[p3r7]

so rn only 2 gamepad models are suported natively (profile bundled w/ norns core).

i made a small norns script that allows one to generate profile from a pluggued in gamepad and output it under ~/we/dust/data/gamepads/.

this PR enables sourcing those user-level profiles dynamically.

2 qs:

• is this a sane approach?
• what about the name / location of the user's gamepad profile (~/we/dust/data/gamepads/)?