Add support for yarn, improve filter test

[OliverUv]

Yarn support for folks who want slightly faster linking 😄

• Includes a simple test for linking with yarn, requires yarn binary in $PATH (npm install -g yarn)
• Should be entirely backwards compatible, both lib and cli. Doesn't even change output unless --yarn is given, in case any user parses output in their CI system.
• Took the liberty of upping the version in package.json to 0.4.0 in accord with semver 2.0
• Lib can be given any executable, but cli will only pass npm or yarn.

[OliverUv]

Looks like that fixed it. Give me a heads up if you want me to squash the last two commits

[OrKoN]

@OliverUv Looks great! I have been working on adding yarn support as well but I found out that yarn link does not install dependencies in the same way npm link does. I don't remember what the problem exactly was so I will try to recover my test files and see if it works with your PR. Thanks 👍

[OrKoN]

@OliverUv which version yarn/npm/node are you using?

[OrKoN]

Interestingly, for me locally the following test constantly fails:

  1) npm-link-shared should install dependencies via linking using yarn:

      AssertionError: lodash does not exist in module-b
      + expected - actual

      -false
      +true
      
      at basic_test (test/basic.js:23:5)
      at Context.<anonymous> (test/basic.js:43:5)

node v6.10.0, yarn 0.21.3, npm 4.4.1

[OliverUv]

Wow, strange! As you can see, the node versions in Travis work (4,5,6). I'm on

~> node --version
v6.10.0
~> npm --version
3.10.10
~> yarn --version
0.21.3

[OrKoN]

Merged and published! Thanks for your contribution.

[OliverUv]

Fantastic, thanks! Did you figure out what the issue was on your side?

[OrKoN]

@OliverUv no, not yet. The only thing that is different is that I am using prefix in .npmrc which points to a folder in the user's home dir. Maybe it somehow affects the linking. Also when I compare the log output I see that on my machine yarn prints a warning that the module-b is already installed.