Sign stellar.sep.7 - URI Scheme

As I understand from:

https://github.com/stellar/stellar-protocol/blob/master/ecosystem/sep-0007.md
https://gist.github.com/nikhilsaraf/ff3ae46116b6ae6dbdcd1743ad9495ec#file-sign_data-go

If I want to sign:
`web+stellar:pay?destination=mortenkjarulff*keybase.io`
The actual string to be signed is:
`printf '\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\4stellar.sep.7 - URI Schemeweb+stellar:pay?destination=mortenkjarulff*keybase.io'`

If this is correct, it would be cool if I could:
`printf '...' | keybase wallet sign --account 'mortenkjarulff*keybase.io'`

Or something more generic.

/Morten

Start by reading SEP-0007 and the linked sign_data-go gist, then locate the existing `keybase wallet sign --account` entry point. Determine the expected input and signing behavior for the `web+stellar` URI, and confirm completion criteria before implementing anything.

Written by the indexing model from the issue text.