Add coverage analysis, Coveralls

[aantron]

Bisect_ppx now supports both native and BuckleScript coverage, so use it.

• [x] make coverage for local reports.
• [x] Submit to Travis, from two build rows (opam and npm).
• [x] Add coverage badge.
• [x] JS: fix [@coverage exclude(file)] upstream, and adapt to that.
• [ ] JS: fix ppx-flags being global, and adapt to that. aantron/bisect_ppx#221
• [ ] JS: fix that instrumentation should automatically only be enabled in dev builds, perhaps also when some flag is specified. aantron/bisect_ppx#221
• [ ] Native: wait for ocaml/dune#57, and adapt to that.
• [ ] JS, esy: relax 4.06 OCaml constraint for development. See https://github.com/aantron/bisect_ppx/issues/220.

[aantron]

On BuckleScript, we need to be able to exclude the test suite.

1. With bs-platform > 6.0.3, it will be possible to pass --exclude-file to Bisect_ppx, which can exclude all test files in one swoop. This is not ideal, as --exclude-file is for working around build system issues.
2. The right way to fix this is to adjust the bsconfig.json schema to be able to specify ppx-flags per directory, so we can specify them in src/ only.
3. For now, we are using [@@@coverage exclude file] in each test file, which is annoying, but less annoying than installing bs-platform master as in (1).

[aantron]

Development with BuckleScript or esy is now locked to OCaml 4.06 and BuckleScript 6.x.x, until https://github.com/aantron/bisect_ppx/issues/220 is resolved.

[aantron]

Blocked on better integration of Bisect_ppx with BuckleScript (https://github.com/BuckleScript/bucklescript/issues/3761).

I disabled coverage on JS for now, because it's too annoying not to be able to use Bisect_ppx as a dev PPX. Coverage is still working on native, because the native library is not published to any package managers.