Handle Syntax Error

First of all thank you for providing this project! It's a great help for my current project.

Regarding this issue: I would like to request that a special case be added that handles syntax errors so that the place of the original source is displayed when there is a syntax error in the generated file.

This is not very useful for projects like coffescript, typescript… which feature their own parsers (and thus do their own syntax error reporting); however it is useful for a use case like mine in which I just copy/past js source from one file into another where the input is not strictly tested for validity.

(I am building a tool that extracts examples from documentation and executes them as tests).

E.g. my errors are correctly resolved for normal exceptions, however when an (induced) syntax error happened, the output pasted below was the output. In there you can see the syntax error indicating the location in the generated file.

This could be solved by adding a special handler for syntax errors; extracting the generated location from the first line of `error.stack`…

```
/tmp/ferrum.doctest-ferrum.doctest-6d12dfba-331b-478f-94bf-862f4569da6/examples.test.js:10 [158/14653]
const foo = 42;
^^^^^

SyntaxError: Unexpected token 'const'
at wrapSafe (internal/modules/cjs/loader.js:1050:16)
at Module._compile (internal/modules/cjs/loader.js:1098:27)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1167:10)
at Module.load (internal/modules/cjs/loader.js:983:32)
at Function.Module._load (internal/modules/cjs/loader.js:891:14)
at Module.require (internal/modules/cjs/loader.js:1023:19)
at require (internal/modules/cjs/helpers.js:72:18)
at /home/karo/pr0j/adobe/ferrum.doctest/node_modules/mocha/lib/mocha.js:334:36
at Array.forEach ()
at Mocha.loadFiles (/home/karo/pr0j/adobe/ferrum.doctest/node_modules/mocha/lib/mocha.js:331:14)
at Mocha.run (/home/karo/pr0j/adobe/ferrum.doctest/node_modules/mocha/lib/mocha.js:809:10)
at Object.exports.singleRun (/home/karo/pr0j/adobe/ferrum.doctest/node_modules/mocha/lib/cli/run-helpers.js:108:16)
at exports.runMocha (/home/karo/pr0j/adobe/ferrum.doctest/node_modules/mocha/lib/cli/run-helpers.js:142:13)
at Object.exports.handler (/home/karo/pr0j/adobe/ferrum.doctest/node_modules/mocha/lib/cli/run.js:292:3)
at Object.runCommand (/home/karo/pr0j/adobe/ferrum.doctest/node_modules/mocha/node_modules/yargs/lib/command.js:242:26)
at Object.parseArgs [as _parseArgs] (/home/karo/pr0j/adobe/ferrum.doctest/node_modules/mocha/node_modules/yargs/yargs.js:1087:28)
at Object.parse (/home/karo/pr0j/adobe/ferrum.doctest/node_modules/mocha/node_modules/yargs/yargs.js:566:25)
```