feat(PATH): add npm bin to path

[kentcdodds]

What:

This will search for a local node_modules/.bin directory and attempt to add it to the PATH (in a cross-platform way using manage-path).

Why:

So people can use these just as they would normal npm scripts. The only case where the PATH doesn't actually contain the npm bin is when you run the p-s script directly (rather than using npm scripts). This is an edge case, but it's still handy

How:

Use find-up to find the node_modules/.bin and mange-path to add it to the PATH in a cross-platform way

Closes #27

[codecov-io]

Current coverage is 100% (diff: 100%)

Merging #38 into next will not change coverage

@@           next   #38   diff @@
=================================
  Files         7     7          
  Lines       137   143     +6   
  Methods       0     0          
  Messages      0     0          
  Branches      0     0          
=================================
+ Hits        137   143     +6   
  Misses        0     0          
  Partials      0     0          

Powered by Codecov. Last update a9a6a62...3ffb5c0

[kentcdodds]

LGTM

This can be merged into next anytime.