Add a platform variant for installing a musl binary

Fix premature exit before the output stream is fully flushed

Drop dependency lodash.isarray

Bump LibSass to 3.4.0

https://github.com/sass/libsass/releases/tag/3.4.0

Update sourceComments documentation in README.md

Use console for logging instead of npmlog

We started using npmlog for install logging in #1786, then went all
out in #1796. npmlog has caused some issues, namely #1805. It doesn't
provided enough value for how troublesome it is.

The problem sparked #1786 was resolved in 5c7fb7f6.

This reverts commit e7ad0cb3.

This reverts commit fa4515d8.

Send npmlog to stdout

Looks the default of stderr is causing issue for some CI servers.

Fixes #1801

I'm not sure why the throw isn't being caught. Probably some async
thing. Don't have time to look into it atm.

Handle permission errors when attempting to cache binaries

This uses a candidate model for determining an suitable cache
directory. This is different from the current approach with eagerly
chooses a cache directory. If that directory isn't accessible for
some reason (i.e. permissions) we have to bail out. With this
approach we can try a prioritised list of candidate directories.

Additionally this address the case where npm cache config is
blank.

Fixes #1781

This reverts commit 5759276e.

Use npmlog for all install script output

In for a penny in for a pound. I also  noticed that #1786 resulted
in some funky looking console output. I've taken this opportunity to
tidy up our install script output whilst maintaining the information
useful for debugging install issues.

```
> node-sass@3.11.3 install /Users/xzyfer/Projects/Sass/upstream/node-sass
> node scripts/install.js

http node-sass install Downloading binary from https://github.com/sass/node-sass/releases/download/v3.11.3/darwin-x64-47_binding.node
http node-sass install Download complete
info node-sass install Binary saved at /Users/xzyfer/Projects/Sass/upstream/node-sass/vendor/darwin-x64-47/binding.node

> node-sass@3.11.3 postinstall /Users/xzyfer/Projects/Sass/upstream/node-sass
> node scripts/build.js

info node-sass build Binary found at /Users/xzyfer/Projects/Sass/upstream/node-sass/vendor/darwin-x64-47/binding.node
info node-sass build Testing binary
info node-sass build Binary is fine

> node-sass@3.11.3 prepublish /Users/xzyfer/Projects/Sass/upstream/node-sass
> not-in-install && node scripts/prepublish.js || in-install
```

Fix binding tests leaking process values