feature request: use custom Lnd framework files

[losh11]

Instead of using a hardcoded Lndmobile.framework (or android equivalent) I think it would be nice if the user could supply their own Lnd framework, which may have custom features, bug fixes, etc. For compatibility maybe you could recommend specific versions of Lnd, or build from a specific commit - this would allow for maximum compatibility whilst allowing external devs to make changes and compile our own Lndmobile.framework.