Support for installing package from git urls

**Is your feature request related to a problem? Please describe.**

This is a feature request.

`apm` is allow us to install package from github/gitlab/bitbucket but `sync-settings` doesn't support that.

**Describe the solution you'd like**

If the package is a git source, use the install source to construct apm arguments.

Here's an example of git package info:
```json
{
"name": "atom-tabs",
"version": null,
"apmInstallSource": {
"type": "git",
"source": "dangh/atom-tabs",
"sha": "9264913b95450bb6ea4ad01338059376a47531ab"
}
}
```
The command I use to install my package from github:
```sh
apm install dangh/atom-tabs
```

The command sync-settings constructed:
```
apm install atom-tabs
```

**Describe alternatives you've considered**

Maybe we can use atom API to install package instead of apm and skipping constructing the command. But I'm not familiar with atom API to say if it's possible to do so.