Add support for multiple install directories

[lorensr]

There are a few people who are trying to use the package for polymer, for which it looks like (see first entry here #31) the bower packages need to be in /public and separately requested by the browser instead of bundled with everything else. However, all packages would then go to that directory, eg /public. It seems like the best solution would be to allow some packages to install to .meteor/local/bower and get bundled and some to go in /public – does that seem to you like the best thing to do? If yes, does anyone have a suggestion for the best way for that config info – which packages go where – to be communicated to meteor-bower? Perhaps added to bower.json in some way that wouldn't conflict with bower itself parsing it, or a separate meteor-bower.json with eg:

{
  ".meteor/local/bower": ["select2", "backbone"],
  "public/bower": ["polymer"]
}

Here is @gbisheimer's suggestion from #41:

I couldn't find a way to use polymer components under .meteor/local/bower folder because polymer components need to have access to each other's files, and you need to have them packaged in meteor during build (using compileStep.AddAsset). You can specify which assets are published by adding a main entry in bower.json (look at select2) and those will get published by meteor-bower automatically.

One possible option can be to use the aditionalFiles entry (deprecated when smart.json support was dropped) in main project bower.json referencing the files that should be included as meteor assets. The code is still alive in meteor-bower but not used anymore. Any other ideas?