Save the source when a mixin is installed

[carolynvs]

When we install a mixin, we need to remember where it came from so that we can upgrade it later without having to specify again where to get the new binaries.

Porter should save this to a new file located at PORTER_HOME/mixins/cache.json.

{
  "mixins": [
    {
      "name": "azure",
      "feedURL": "https://cdn.deislabs.io/porter/atom.xml"
    },
    {
      "name": "mymixin",
      "url": "https://github.com/org/proj/releases/downloads"
    }
  ]
}

feedURL and url correspond to the original flags passed to porter mixin install, which can be either --feedURL or --url.

I suggest looking at implementing this functionality in this area:

https://github.com/deislabs/porter/blob/b24cdd412147c8156942f6d8accb489e71e48e6f/pkg/mixin/provider/install.go#L17-L23

It has a reference to a Config struct which has useful methods like getMixinsDir() which you will need in order to figure out where to write the file.

You can test this out by building and installing porter, then running porter mixin install exec. Then check the file in ~/.porter/mixins/cache.json.