add goja as JS VM for transformers

[jipperinbham]

goja looks to be a nice alternative to offer people who may need a little more performance from the transformer function. Initial benchmarks showed the following results:

$ benchcmp otto.profile gojajs/gojajs.profile
benchmark                   old ns/op     new ns/op     delta
BenchmarkTransformOne-8     13565         6265          -53.81%

• CHANGELOG.md updated
• README.md updated

fixes #230

[codecov]

Codecov Report

Merging #294 into master will decrease coverage by 0.09%.
The diff coverage is 88%.

@@            Coverage Diff            @@
##           master     #294     +/-   ##
=========================================
- Coverage   80.84%   80.74%   -0.1%     
=========================================
  Files          46       48      +2     
  Lines        2490     2587     +97     
=========================================
+ Hits         2013     2089     +76     
- Misses        335      350     +15     
- Partials      142      148      +6

Impacted Files	Coverage Δ
adaptor/transformer/ottojs/client.go	45.71% <ø> (ø)
adaptor/transformer/ottojs/writer.go	68.81% <ø> (ø)
adaptor/transformer/transformer.go	100% <100%> (ø)	✅
adaptor/transformer/gojajs/client.go	82.35% <82.35%> (ø)
adaptor/transformer/gojajs/writer.go	89.65% <89.65%> (ø)
state/filestore.go	49.05% <0%> (-3.78%)	❌
adaptor/postgres/tailer.go	78.97% <0%> (-1.14%)	❌

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