Add support for lnurl pay

[saubyk]

Add ability to pay to a lightning address or a lnurl pay string.
Lightning Address (and lnurl) has achieved wide spread adoption across the lightning wallet ecosystem and is missing on RTL. It's critical to add this functionality to given the users the option to pay via this popular standard.

[ShahanaFarooqui]

@saubyk Why don't you release v0.15.0 in the mean while :-p

[ghost]

LNURL integration would be awesome, since we could then use the "donate" button from within RTL, which redirects us to RTL.info/donate, but only spits out a LNURL pay option lol