Support latest Go version

[bzz]

TODO

• new CI profile for 1.13
• rebase after Change URL #2
• see the tests also failing on CI on go 1.12 ddcf524
• fix the unexpected tokenizer test failure

[bzz]

CI failure on tokenizer test has to do with how append works on regex matches, with are slices of bytes sharing the same array underneath, and is the exact cause of src-d#196

[codecov-io]

Codecov Report

Merging #4 into master will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
- Coverage   67.66%   67.63%   -0.03%     
==========================================
  Files          22       22              
  Lines        1101     1100       -1     
==========================================
- Hits          745      744       -1     
  Misses        297      297              
  Partials       59       59              

Impacted Files	Coverage Δ
internal/tokenizer/tokenize.go	97.50% <100.00%> (-0.04%)	⬇️

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