Add ability to use `interrupt::enable` and `interrupt::clear` on arbitrary GPIO/EXTI lines

[interrupt::enable and interrupt::clear](https://github.com/rubberduck203/stm32f3-discovery/blob/33db991148879278bc3ce6404bf68a47bf81fc01/src/button/interrupt.rs) are currently hard coded to work only for the user button.
Allow these to work for any interrupt the user may want to use on the board.

Also, add `UserButton.interrupt_enable()` and `UserButton.interrupt_clear()` methods?