[CLI] CLI should look at cwd and use locally-installed binary (if present)

[jeffmo]

One problem with npm install -g is that it installs an executable at a certain version -- whereas various projects could be built using/expecting a different version.

One way I dealt with this for the Jest CLI was to look at CWD and try to determine if we're currently inside a project that depends on the CLI (and, if so, delegate execution to the local version instead of the globally-installed version).

This should also help reduce the frequency with which people need to update their global CLI version.

[bassettsj]

Agreed. This is why grunt-cli exists.

[ryyppy]

Are you talking about the flow-typed cli binary?

[jeffmo]

@ryyppy: right

[jeffmo]

(PS: I have this PR mostly ready, just need to test a bit more before I send it out for review)