Search for ember project and don't enable commands without finding one

[nigelsmith]

Hi,

Regarding issue #14 this is one possible solution:

1. if the user has set a path then always prefer that
2. if the user hasn't set a path then recursively search for an ember-cli directory; here I've opted to look for the ".ember-cli" file which ember creates (ofc if they should remove that for some reason then that would have to change, though it should be faster than node requiring a file)

If a working directory is found then add the commands to the command palette. It's hard to tell if directory structure is the only reason that people reported some of the issues, especially since there are some reports of restarting fixing the problem, but this ought to be of general use anyway.

Many thanks for the plugin.