Git raw command

I've added a command to run "raw" git commands. User can define whatever command they like. It's like the `Custom Command` command, but using `Default.sublime-commands` file. It allow for faster access to commands and avoid typing again and again the command problem with "custom command".

Example of a `Default.sublime-commands` file:

```
{
    "caption": "Git: show git log in default viewer",
    "command": "git_raw",
    "args": {"command": "git log --oneline"}
}
```

Also, it's possible to choose where to show the result of the command (the default value is `pane_below`):

```
{
    "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"}
}
```

Note: this commit is based on the python3 branch, so I've only tested it with Sublime Text 3 (build 3059).