Add initial test262 support to test optional chaining

[alangpierce]

Progress toward #461

For now, we only run two subdirectories and ignore various tests (sloppy mode,
tests expecting syntax error, and TCO tests). This reveals a few issues with the
current implementation, which will be addressed later. Since it's not a passing
build yet, I'll leave it out of CI for now.

Also drop the Travis build for Node 8 since one of the new dependencies requires
Node 10. I won't officially drop Node 8 support just yet, but will soon, and
it's at end-of-life now.

[codecov-io]

Codecov Report

Merging #494 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #494      +/-   ##
==========================================
- Coverage   82.56%   82.52%   -0.05%     
==========================================
  Files          54       54              
  Lines        5536     5304     -232     
  Branches     1236     1236              
==========================================
- Hits         4571     4377     -194     
+ Misses        678      643      -35     
+ Partials      287      284       -3

Impacted Files	Coverage Δ
src/Options-gen-types.ts	50% <0%> (-50%)	⬇️
src/util/getDeclarationInfo.ts	80% <0%> (-20%)	⬇️
src/util/elideImportEquals.ts	81.81% <0%> (-18.19%)	⬇️
src/parser/index.ts	77.77% <0%> (-7.94%)	⬇️
src/computeSourceMap.ts	83.33% <0%> (-5.56%)	⬇️
src/util/getNonTypeIdentifiers.ts	94.44% <0%> (-5.56%)	⬇️
src/parser/plugins/types.ts	91.66% <0%> (-3.34%)	⬇️
src/parser/tokenizer/readWord.ts	82.35% <0%> (-3.02%)	⬇️
src/util/shouldElideDefaultExport.ts	75% <0%> (-2.78%)	⬇️
src/parser/traverser/util.ts	86.2% <0%> (-2.17%)	⬇️
... and 18 more

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