Fix proxy config for binary download

In #1725 we started setting `proxy: ''` to defer proxy resolution
logic to request. However `request` uses `hasOwnProperty` rather
than falsey checks to determine if an option has been set. This
caused `request` to believe were configuring a proxy so it didn't
do it's own proxy resolution.

Fixes #1785
Fixes #1787

Update supported node versions

#1766

This reverts commit b6558523.

Fix links in troubleshooting.md

Respect NO_PROXY when downloading the binary

Turns our handling proxies is complicated. The sanest thing to do
is try matching [npm's behaviour][1], and liable [to change][1] to
our benefit. The TLDR; of which is to let `request` do whatever
it wants unless npm has been explicitly configured otherwise.

Fixes #1724

[1]: https://github.com/npm/npm/commit/40afd6aaf34
[2]: https://github.com/npm/npm/issues/7168

Fix process argument binary configuration example

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"