Improve source map support

[kpdecker]

Attempt to resolve #274.

This adds a couple of different features:

1. Source maps are now aware of existing source maps and will merge these with the istanbul generated source map.
2. Adds option to include source maps inline in the instrumented output.
3. Adds source map error support under node when using run-with-cover.

Net effect is this:

  1) parser parsers partial blocks:
     TypeError: Cannot read property 'line' of undefined
      at Object.prepareProgram (./dist/cjs/handlebars/compiler/helpers.js:9:8043)
      at Object.anonymous (./dist/cjs/handlebars/compiler/parser.js:9:3727)
      at Object.parse (./dist/cjs/handlebars/compiler/parser.js:9:37453)
      at HandlebarsEnvironment.parse (./dist/cjs/handlebars/compiler/base.js:9:3159)
      at astFor (./spec/parser.js:7:26)
      at Context.<anonymous> (./spec/parser.js:117:12)

From this:

  1) parser parsers partial blocks:
     TypeError: Cannot read property 'line' of undefined
      at Object.prepareProgram (lib/handlebars/compiler/helpers.js:135:17)
      at Object.anonymous (lib/handlebars/compiler/parser.js:16:20)
      at Object.parse (lib/handlebars/compiler/parser.js:263:36)
      at HandlebarsEnvironment.parse (./dist/cjs/handlebars/compiler/base.js:9:3159)
      at astFor (./spec/parser.js:7:26)
      at Context.<anonymous> (./spec/parser.js:117:12)

This could use additional test coverage for the source-map-support integration, but I wanted to get this out there for feedback before investing too much time in that implementation.

[kpdecker]

Not a fan of this being handled differently vs. the other post hooks, but there was a need to access the instrumenter instance and changed the hook API seemed like it would be even more problematic.

[kpdecker]

The run-with-cover implementation had a timing issue that could result in source maps being incorrectly applied to files. I've reworked this and rebased to avoid this issue.

[tilgovi]

This is pretty nice. It's very similar to what this did for karma-coverage.

I think it needs to be combined with what's on the source-map branch right now.

I'm using karma and browserify with an ES6 Babel project so I use browserify-istanbul. Since babelify is handling the first step and outputing inline source maps (like all browserify transforms), istanbul needs to read the inline source map and output an inline source map.

• With this PR, the inline source map to be rewritten, but the coverage report is not transformed properly and the line coverage counts look off.
• With the source-map branch, the inline source map is not rewritten, but the coverage report is correct.

Combining these I think we could have correct coverage and source maps in the browser when debugging tests that are built with browserify.