Default apps to open files

- finds available programs for mime types
- saves and restores defaults from json file
- if no default and only one available app, it uses the app
- if there is multiple choices or only apps for any mime available, it
lets the user choose
Apps run in child processes
Merge branch 'master' of github.com:zodern/Silk
merged with app-defaults
defaults with for framework
moved defaults to its own app
small fixes
headless fix
that shouldn't be there
Merge branch 'master' of github.com:zodern/Silk
Merge pull request #8 from formula1/master

Apps run in their own child Processes