Add rule to lint trailing commas in select clauses

[alanmcruickshank]

This fixes #362.

[codecov]

Codecov Report

Merging #752 (0d1c926) into master (4ef3139) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #752      +/-   ##
==========================================
+ Coverage   96.03%   96.04%   +0.01%     
==========================================
  Files          95       96       +1     
  Lines        6756     6779      +23     
==========================================
+ Hits         6488     6511      +23     
  Misses        268      268              

Flag	Coverage Δ
dbt017-py38	60.31% <37.50%> (-0.09%)	⬇️
dbt018-py38	60.34% <37.50%> (-0.09%)	⬇️
py36	95.24% <100.00%> (+0.01%)	⬆️
py37	95.24% <100.00%> (+0.01%)	⬆️
py38	95.19% <100.00%> (+0.01%)	⬆️
py39	95.19% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
src/sqlfluff/core/rules/config_info.py	100.00% <ø> (ø)
src/sqlfluff/core/rules/base.py	88.83% <100.00%> (+0.22%)	⬆️
src/sqlfluff/core/rules/std/L038.py	100.00% <100.00%> (ø)

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[dmateusp]

I didn't know some platforms allowed it! It's a scary world out there 😄

[alanmcruickshank]

I didn't know some platforms allowed it! It's a scary world out there 😄

BigQuery allows it but as far as I know, they're the only ones. That's why I think it's a safe bet to forbid.