Add @kmer_str macro to allow kmer string literals

[kdm9]

Also adds documentation and a doctest.

[codecov-io]

Codecov Report

Merging #446 into master will increase coverage by 0.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #446      +/-   ##
==========================================
+ Coverage   71.58%   71.59%   +0.01%     
==========================================
  Files         138      138              
  Lines       10271    10272       +1     
==========================================
+ Hits         7352     7354       +2     
+ Misses       2919     2918       -1

Impacted Files	Coverage Δ
src/seq/Seq.jl	0% <ø> (ø)	:arrow_up:
src/seq/kmer.jl	86.2% <100%> (+0.16%)	:arrow_up:
src/align/alignment.jl	86.13% <0%> (+0.99%)	:arrow_up:

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[TransGirlCodes]

Can the doctest be added as an actual unit test in addition?

[kdm9]

Should be good to go now @Ward9250

[bicycle1885]

Can you mention this macro in NEWS.md?

[bicycle1885]

Thank you, @kdmurray91!