make CEF cache location settable in gyp file / config script

[gruehle]

In order for this shell to be easily used by multiple applications, we need each application to be able to configure where its CEF cache is stored. Right now, this has to be done by modifying the C++ files.

This should probably be settable in the gyp file and/or config script. Alternatively, an app name could be specified in the gyp file / config script, and that name could then be used as part of the CEF cache path.

Right now, we have it hardcoded to "Brackets" on Windows, and hardcoded to use the bundle identifier on mac. The latter is probably okay. But, if we make it parameterizable on Windows, it should work the same on both platforms (so we'll need to STOP using the bundle identifier on mac and instead use the parameter).

[gruehle]

Migrated from brackets-shell, issue 51