Custom external tools

[mattstermiller]

VS-style custom tools. Allow the user to maintain a list of programs as commands.

Each tool has:

• Name
• Exe path
• Arguments
• Item types checkboxes:
  • Files
  • Folders

Variable support:

• Exe path and Arguments can contain environment variables (notation TBD)
• Arguments can contain Koffee variables:
  • Current directory
  • Path of item under cursor
  • Paths of selected items

This feature would replace the existing commands Open Commandline and Open With Text Editor. This first version will not have a UI but only read from the config file. Write instructions for configuration as a separate markdown doc.