Custom PrefixByte Support

[lxfontes]

Allow programs to introduce their own prefixes, avoiding collision with known prefixes.
Scenario: Signing binary 'modules' using pub keys prefixed with M.

var modulePrefix PrefixByte = 12 << 3 // Base32-encodes to 'M...'

// register new prefix
nkeys.AddPublicPrefix(modulePrefix, "module")

// creating a new pair
moduleKey, _ := CreatePair(modulePrefix)

// prefix name Stringer returns 'module' instead of 'unknown'
modulePrefix.String()

// use .PrivateKey(), .PublicKey(), .Sign(), .Verify() as usual

The resulting data looks like:

Seed SMAPILBZ447DMKKISBEUPTV4M6OMLGPSGSZDWEYRDH3OPSCFW2HG5H5CNQ
Pub MDDWNXZNY4OM2EFCQO7MOPOS4SGHOVBNV5GZ53T3LWHMHRSN5VFDFN6Y
Private PD2CYOPHHY3CSSEQJFD45PDHTTCZT4RUWI5RGEIZ63T4QRNWRZXJ7R3G34W4OHGNCCRIHPWHHXJOJDDXKQW26TM65Z5V3DWDYZG62SRSHSQQ