feat: Support for local files

[vHeemstra]

Support for copying local binaries before trying the downloads.

Works the same as .src(), except url is a local filepath.
Bin-wrapper now tries the localSrcs first (copying them in place when found) and falls back to downloading srcs on fail.

Reason

Downloading binaries slows down CI scripts significantly and a lot of the packages using bin-wrapper ship with binaries which have already been downloaded and can be used.

Changes

• Added local file support
• Updated docs

[vHeemstra]

Note: Travis CI test fail, because current xo and ava are different from originally used ones. To solve this, this would need the PRs updating the code base to be merged.

[bjesuiter]

@vHeemstra This is cool, bc. I need a similar thing.
I'm behind corporate firewalls and using this bin-wrapper with network access ragens from 'pain' to 'impossible'.
So I'm constantly wrapping binaries I need in npm packages and deploying them to npm.

I've one question regarding your code: Do you really need to copy the dependency?
Since I want to use it with the binaries distributed inside the same package, I would not need them to be copied, as this would only increase the size of my already big package.

What I want to create are basically "offline-installers", where everything is contained in the npm package. (Since these would be cached on our corporate internal Nexus package manager) :D