Make Hosts Extensible / Add Ability to Customize Hosts via Extensions

[#1]

There appears to be an issue using certain systems as sFTP hosts, and some systems that may be desirable or suitable to act as a host support REST APIs but not sFTP (e.g. Marketo). Please add the ability to define new host configurations via extensions so that extensions can be created to support various environments in which libraries could be hosted.

No assumptions should be made with respect to how files should be sent to the host. For example, Marketo doesn't support real folders, so it's not possible to use Launch's native folder/file structure present in zipped libraries.