Better cross platform support

[dersam]

There are several components in prophet that assume Linux-like commands are available (the plugin system is the most obvious culprit). Refactor this to be better abstracted for use on Windows.

[dersam]

Using symfony/filesystem and symfony/process will simplify this.

[dersam]

May no longer be relevant if Bash for Windows has staying power. Better abstraction of cli calls still good idea though.