add the ability to execute a custom command

Example usage: put the following JSON in a ".sublime-commands" (or ".sublime-menu") file:

{
    "caption": "Git: Gitk all branches",
    "command": "git_raw",
    "args": {"command": "gitk --all"}
}
add option to choose where to display git raw command output

Example usage: put the following JSON in a ".sublime-commands" (or ".sublime-menu") file:

{
    "caption": "Git: show git log in default viewer",
    "command": "git_raw",
    "args": {"command": "git log --oneline"}
},
{
    "caption": "Git: show git log in a pane below",
    "command": "git_raw",
    "args": {"command": "git log --oneline", "show_in": "pane_below"}
},
{
    "caption": "Git: show git log in the quick panel",
    "command": "git_raw",
    "args": {"command": "git log --oneline", "show_in": "quick_panel"}
},
{
    "caption": "Git: show git log in a new tab",
    "command": "git_raw",
    "args": {"command": "git log --oneline", "show_in": "new_tab"}
}
Merge pull request #315 from pomeh/git-raw-command

Git raw command