Plug-in & configuration system enhancements

[curlpipe]

• [x] Default plug-ins that are always enabled regardless of user's system
• [x] Rename ` tospacein key bindings (but allow ` as well)
• [x] Better document type in status line (to prevent it being empty)
• [x] Configurable help message and smaller greeting message by default (#147)
• [x] Allow more substitution options on help message and greeting message
• [x] Better plug-in and configuration errors
  • [x] better errors being fed to main.rs
  • [x] scan through editor API calls and see if there is a way to fire an error there
  • [x] try different things to see if all errors are reported appropriately
• [x] Plug-in management system (installing, removing, checking the status of)
• [x] More advanced plug-in API
  • [X] Allow binding to events before the editor applies the changes based on that key press
  • [x] Networking (HTTP get requests)
  • [x] Request re-rendering of certain parts of the editor
  • [x] Have a system like "run function f every n seconds"
  • [X] Can render plug-in output in certain areas
    • [X] greeting message
    • [X] help message

[curlpipe]

Completed with the release of 0.6.3