feat: support custom expression precedence

[devongovett]

A custom generator can be passed to astring to extend it with additional nodes. We are using this in Parcel to convert Babel's AST format to ESTree on-the-fly. However, one limitation is that expression precedence for these node types is unknown to astring. This PR adds support for passing a custom expressionPrecedence map to astring which can extend from the default mapping (which is now exported) in order to support this use case.

[codecov]

Codecov Report

Merging #427 (db7c2fd) into master (6021bf0) will increase coverage by 0.28%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #427      +/-   ##
==========================================
+ Coverage   95.44%   95.72%   +0.28%     
==========================================
  Files           1        1              
  Lines        1119     1123       +4     
==========================================
+ Hits         1068     1075       +7     
+ Misses         51       48       -3     

Impacted Files	Coverage Δ
src/astring.js	95.72% <100.00%> (+0.28%)	⬆️

---

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 6021bf0...28c8865. Read the comment docs.

[davidbonnet]

Thanks @devongovett for suggesting this.
Note that I renamed the option to:

expressionsPrecedence
          ^