It is not clear from the `--help` docs that `--source-map` requires a boolean or path. Passing the flag alone results in a TypeError. There's an argument to be made that passing the flag alone should imply `true`, or at the very least it should fail with a helpful error, not a cryptic TypeError. For now, this PR fixes the --help docs so that they match the README.

with relative path to troubleshoot guide, npm was unable to direct to correct page. Now its corrected using absolute path.

See https://github.com/sass/libsass/releases/tag/3.5.4

Revert to sass-spec@3.5.4-1 because of specs that depend on features
reverted in 3.5.4.

Raw CSS imports have been removed from LibSass. We need to opt into
the feature with `sass_option_push_import_extension`. We're must do
this to maintain BC. Any imported files with a `.css` extension will
produce a deprecation warning.

This commit should be reverted in v5. This would remove raw CSS
imports once and for all.

This reverts commit 8004d107.

This is an extension of #2268. Catches a few places missed the
first time around.

* Bump LibSass@3.5.3

See https://github.com/sass/libsass/releases/tag/3.5.3

* Workaround sass-spec

A patch landed in sass-spec that is not compatible with 3.5.

This is an alternative to some of the fixes in [1].

Starting with Nan 2.9.0, we have the ability to propagate async context
across async hops. Certain variants of Nan::Callback::Call are now
deprecated to encourage use of the context presevering alternatives.
Certain variants of Node's MakeCallback that were used internally are
going to be deprecated in Node 10.

Summary is that one should use Nan::Call for sync calls, and
Nan::Callback::Call for async. The latter expects an async resource
corresponding to the async operation to be provided at the call time.

This patch fixes things up so that 1) node-sass isn't using any
deprecated APIs, and 2) properly propagates async context for async
callbacks by creating async resources in the appropriate places.

[1]: https://github.com/sass/node-sass/pull/2295

Work around https://github.com/nodejs/nan/issues/755

Fixes #2287

Fixes #2280

Due to #1912 we were writing binaries to disk with the encoding.
We need to do some additional juggling when dealing with binary
data.