Added initializer for specifying own random data

[mufumade]

This pull requests adds a initializer to ULID for specifying the date as well as own random data like so:

let ulid:ULID? = ULID(timestamp: timestamp, uuid: Data(someData))

• In the case that the given data is too short then nil is returned.
• If the given data is too long then the excess data is discarded.

[codecov-commenter]

Codecov Report

Merging #6 (b0b7d99) into master (829acd3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master       #6   +/-   ##
=======================================
  Coverage   98.95%   98.95%           
=======================================
  Files           4        4           
  Lines         574      576    +2     
=======================================
+ Hits          568      570    +2     
  Misses          6        6           

Impacted Files	Coverage Δ
Sources/ULID/ULID.swift	100.00% <100.00%> (ø)
Tests/ULIDTests/ULIDTests.swift	98.40% <100.00%> (+0.16%)	⬆️
Sources/ULID/Data+Base32.swift	97.27% <0.00%> (-0.31%)	⬇️
Tests/ULIDTests/Data+Base32Tests.swift	100.00% <0.00%> (ø)

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[yaslab]

Thank you for your pull request!

I think this is a useful feature, so I'll merge it. However, I think that randomPartData is more appropriate than uuid for the second parameter name, so I'll fix it.