Add cross-platform support for pre/post version script hooks.

[webuniverseio]

Also fix error reporting for exec function that was failing when result stdout and stderr are null (for example in case when ENOENT error is surfaced). Fixes: #2

[webuniverseio]

@BigstickCarpet , what do you think?

[JamesMessinger]

@szarouski - Thanks for the PR. It looks good, but if you could add a couple more things, I'd really appreciate it.

Script order
To fully reproduce the behavior of npm version, it should perform the steps that are listed here. Specifically:

1. Run the preversion script before the version is updated (and before the version prompt is shown)
2. Run the version script after the version is updated, but before git commit and git tag
3. Run the postversion script after git commit and git tag, but before git push

Tests
Please add a few tests for this new functionality. The tests currently work by modifying the PATH variable to run a dummy git binary that just writes its arguments to a text file. Then this function is called to assert that git was run with the expected arguments. The same approach should be taken with npm, so we can assert that npm is being called as expected.

[webuniverseio]

@BigstickCarpet Ok, I'll try. First I had to make tests run on windows: https://ci.appveyor.com/project/szarouski/version-bump-prompt. I added appveyor CI (with a little bit of pain). Would you mind dropping support for node < 4? I can make it working, but it require more time and I'm not sure it is much needed. Worst case people can submit another PR.

[webuniverseio]

@BigstickCarpet Please review again.

[JamesMessinger]

@szarouski - Thanks for all your work on this. I haven't had a chance to review your latest commits yet (been too busy at work), but should be able to look at them next week. Sorry for the delay :(

Thanks for adding appveyor CI. I've been meaning to do that anyway. 👍

I'm fine with dropping support for old Node versions (< 4.0). Just update the engines settings in the package.json file accordingly.

Thanks again!

[webuniverseio]

No problem, I'm happy to contribute 😄. I updated engines setting.

[JamesMessinger]

@szarouski - I've finally got some free time, so I'm merging this PR now. Will make some additional changes and then publish it to npm as v3.0.0

[JamesMessinger]

@szarouski - v3.0.0 has been published to npm. Thanks again for your help! I gave you credit in the changelog 👍