Added transacting ability.

[sushshring]

This allows select statements to run with the FOR SHARE and FOR UPDATE locking reads. Queries performed in this way ensure that no data is inserted/updated during the read, preventing race conditions.
To access this functionality, use

squel.select().transact(forUpdate: boolean)

[codecov]

Codecov Report

Merging #361 into master will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #361     +/-   ##
=========================================
- Coverage   81.31%   81.22%   -0.1%     
=========================================
  Files           2        2             
  Lines        2863     2881     +18     
  Branches      635      639      +4     
=========================================
+ Hits         2328     2340     +12     
- Misses        535      541      +6

Impacted Files	Coverage Δ
dist/squel-basic.js	93.27% <0%> (-0.12%)	⬇️
dist/squel.js	72.19% <0%> (-0.1%)	⬇️

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