Should be --sass-binary-site instead of --SASS_BINARY_SITE

This was removed #1733 but was depended on by #1714.

User configurable cache folder for binary download

Adding unit test coverage to install

The installer is getting more complicated. The core of it is a http
request which makes testing it difficult/messy. Pulling out core
logic into small util functions that we can unit test would give me
much more confidence moving forward.

I would prefer we don't export functions from scripts so people don't
start relying on undocumented APIs. This is why I opted for helper
utils but I'm not committed to them.

New order of operations
1. Look for existing binary in vendor folder
2. Create target vendor folder
3. Look to see if we’ve cached a copy in the configured or NPM cache
4. Download to current version’s /CACHE/node-sass/version/binding_file
5. Copy the cached download to the regular vendor directory
Closes #1566

Node 5 is neither latest or LTS. This is not removing support for
Node 5, it's simpky removing it from day-to-day CI for faster builds.

This originally caused issues because sass spec was in the test
folder. As of #1698 sass spec is now an npm devDependency. Without
the test folder published the node-citgm cannot execute our
`npm test`.

These specs would use the local environment for `SASS_BINARY_NAME`.
This means in the local environment wasn't supported these specs
would fail unexpectedly. This doesn't play well with the node-citgm
because these specs would fail anytime the module version was bumped.

Update ISSUE_TEMPLATE with Bootstrap format

Took the basic structure from Bootstrap since they have a large project
that gets a lot of issues
[skip CI]

Replace util._extend with Object.assign

Update spec runner for full sass-spec coverage

Closes #1709

ESLint 3.x updates

This was never part of the public API, and has officially been
depreciated in Node 6 https://github.com/nodejs/node/pull/4903
Object.assign can't be used yet since it requires ES2015

Revert "Publish tests to npm"

This reverts commits
2c2f6a24
3bc61658
23f3db32
4dcf37bf

Previously the runner only picked up the first level of the test suites.
Now we pick up each folder as a Mocha suite, which increases the tests
run from ~1K to 6K.

Failures in after_success don’t actually fail the build.

Turns out old Java API struggles with certain UTF-8 characters.

https://github.com/dlmanning/gulp-sass/issues/529
https://github.com/sass/node-sass/issues/1699

This is affecting JVM based build systems like Jenkins

https://issues.jenkins-ci.org/browse/JENKINS-12610
https://issues.jenkins-ci.org/browse/JENKINS-33478

Remove the cli before test

This test is super flakey for some reason, and `before` hooks also
appear to be get retried by mocha.

The was only required for when testing against LibSass < 3.3 which
is no longer the case. As such this hook serves no purpose.

Don't run tests when building a Windows release

Fixes to troubleshooting doc

package.json will not be created without npm init and a --save is required to append the installed version of package onto the package.json

Move the two CLI Usage into separate lines