Add command to return the variables used in a .fw file

[fhunleth]

People have been using variables to pass provisioning information through fwup so that programming and provisioning can be done in one step. The way this is done is to escape environment variables in fwup.conf files so that they get evaluated the the .fw is applied rather than when it's created.

There's a desire to get the list of variables referenced in a .fw file for the following reasons:

1. See what's being provisioned without unzipping the .fw and looking at the meta.conf
2. Getting a list of variables so that scripts can pass the variables through the sudo call to fwup without knowing the variable names. sudo resets the environment. This is for generic scripts like those used in Nerves.