Add postgres json literal

[wexder]

Json types are crucial for postgres. This PR add support to easily cast golang string as postgres json type.

Solves #160.

[codecov-commenter]

Codecov Report

Merging #161 (11b0a68) into master (38b6caf) will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #161      +/-   ##
==========================================
- Coverage   92.60%   92.58%   -0.03%     
==========================================
  Files         115      115              
  Lines        6846     6852       +6     
==========================================
+ Hits         6340     6344       +4     
- Misses        405      407       +2     
  Partials      101      101              

Impacted Files	Coverage Δ
postgres/literal.go	92.30% <66.66%> (-3.35%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 38b6caf...11b0a68. Read the comment docs.

[go-jet]

Thanks, looks good.