typescript+babel and coverage improvements

[fasterthanlime]

See #76

• Drop istanbul@0.4.5 dependency, add babel-plugin-istanbul
• Add getEnv() helper to SimpleCompilerBase
• Support an environment string in the coverage option for both typescript and babel compilers
• Make babel compiler use compilerContext as compile options
• Drop sorcery dependency, pass inputSourceMap to babel instead
• Always use babel+babel-plugin-istanbul to instrument code for measuring coverage
• In ts+babel, translate ts sourcemap options to babel. With a twist, for
  inline sourcemaps.

[fasterthanlime]

Quick update: using context to pass inputSourceMap breaks the inline html compiler. Fixing now

[fasterthanlime]

Quick update: using context to pass inputSourceMap breaks the inline html compiler. Fixing now

Fixed in 262476a

[fasterthanlime]

Another quick change I'm about to make: forward coverageInclude and coverageExclude keys to babel-plugin-instrument.

It's not possible to use the nyc block in package.json method unfortunately, because inputSourceMap is already passed directly to babel-plugin-instrument, so it doesn't even look at the package.json.

[fasterthanlime]

Latest semantics:

• code is only ever instrumented in testing (if you can think of a use case where one would like code to be instrumented in a non-test environment, let me know).
• coverage: true does the same thing as before (except only in test)
• coverage: { /* opts */ } passes options to babel-plugin-istanbul, like include and exclude, see documentation in fasterthanlime/electron-compile@4b23237

[fasterthanlime]

New find! While babel:

• will accept an inputSourceMap
• will merge/combine/remap sourcemaps correctly
• does have a sourceMaps: "inline" option

...if you specify inputSourceMap, sourceMaps is forced to true rather than "inline", so if you enable coverage on my branch, you don't get inline source maps. Fixing that now.