Parse like and ilike clauses as comparisons

[dbczumar]

This PR addresses #493 by introducing a regex filter to parse the following tokens as comparison operators, rather than keywords: LIKE, ILIKE, NOT LIKE, and NOT ILIKE. These terms are regarded as operators by popular SQL engines (e.g. Postgres, MySQL).

@andialbrecht There is a backwards-compatibility consideration here: reclassifying these terms as comparison operators may break preexisting user code that relies on the designation of the terms as keywords. Please let me know what your thoughts are here.

[andialbrecht]

@dbczumar thanks! The change looks good in general. Could you have another look at the test you've added? Travis reports a failure just for this test: https://travis-ci.org/andialbrecht/sqlparse/builds/629594200

[dbczumar]

@andialbrecht Sorry for the delay - I've fixed the test (it required a bit of special casing in the test's validation logic because Comparison tokens do not define a ttype). Let me know how it looks!

[codecov-io]

Codecov Report

Merging #525 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #525   +/-   ##
=======================================
  Coverage   96.95%   96.95%           
=======================================
  Files          21       21           
  Lines        1543     1543           
=======================================
  Hits         1496     1496           
  Misses         47       47

Impacted Files	Coverage Δ
sqlparse/keywords.py	100% <ø> (ø)	⬆️

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[andialbrecht]

Looks good, thanks a lot! This kind of "deep inspection" into the queries always requires some extra stuff... :)