Add ability to specify suffix for npub

[jonathanwxyz]

Fixes #11

Users can now specify an -s flag to search for a given list of suffixes. This works alongside the -n flag, so users can specify both a prefix and a suffix at the same time.

[gu1p]

Before rana start mining, the message "Started mining process for vanity bech32 prefix[es]: ... (estimated pow: 12)" is printed. I think you should complement that with the suffix part.

[grunch]

tACK

It works great, thanks for this contribution 🥳