Closes #2027

Fixes #1952
Closes #1956

- fixes issues with transitive dependency scss-tokenizer
- allows graph to follow symlinks

This logic is all tightly coupled in the bin. This logic has been
notoriously impossible to test due to weird fs timing issues. By
extracting the actual logic we're now able to test it in isolation.

With this in place replacing Gaze (#636) becomes a viable option.
This PR not only massively increases our test coverage for the
watcher but also address a bunch of known edge cases i.e. orphaned
imports when a files is deleted.

Closes #1896
Fixes #1891

Readme: Fix typo in alt text of Slack Badge

Add Electron ABI versions

This adds the respective Electron ABI versions to `getHumanNodeVersion()`
to enable running `node-sass` under Electron where the ABI versions differ from Node's.

* Move Contributing.md under .github
* Move ISSUE_TEMPLATE.md under .github

* Chore: Move ISSUE_TEMPLATE under .github

Closes #1983

Run the initial listing before running the full matrix of builds

Update module version for Node 8

TODO(xzyfer): fix this post Node 8

Update module version for node v8.x

V8 5.7 landed recently in node core, causing the module version to get bumped.

Add Node 8 to the support versions lists

Use hardcoded process values rather than inspecting the current
environment.

This is a workaround for nodejs/citgm#389. The install/build flow
needs to be updated so that unsupported environments result in an
informative warning instead of a failure.

Closes #1838
[skip ci]

* Update request version to avoid npm warn

Fix npm rebuild

Running `npm rebuild node-sass` as we instruct people, has been
noop for a while. I'm not entirely sure what has changed.

Debugging as reveal that running `rebuild` now just triggers the
`install` and `postinstall` scripts. The `build` script is no
longer run. This suggests something has changed in newer versions
of npm.

Since the `--force` flag is required to rebuild the binary I've
update the `build` script to take into account if `--force` was
passed to npm.

Restore behaviour of cwd in include path

Traditionally Sass has added the cwd as the first include path. This
behaviour was remove in Sass 3.4, and recently in LibSass 3.5.0.beta.2.
People depend on this behaviour and as a result a lot of compilations
are now failing.

This PR restores the expected behaviour and adds a test to boot.

Fixes #1876

Update AppVeyor release config

The AppVeyor config was updated in #1870 but I missed some changes
for the release config.

Make the watcher more responsive to child changes

Watching for changes on files with lots of `@import`s had a
significant regression in responsiveness in #1745.

The regression was caused by calling `gaze.add` unnecessarily. We
only need to call `gaze.add` on files that aren't currently being
watched.

At the time I confirmed that calling `gaze.add` in files that were
being watched wouldn't result in a leak or multiple events being
fired. I however assumed calling `gaze.add` for already watched
files would be very cheap.

Fixes #1869