added `launchCommand` option

### `--launchCommand="your_command_here %U"`

Specify a custom command to launch. `%U` will be replaced with the URL
 that corresponds with the working git directory.

NOTE: This will execute *before* opening the browser window if
 the `launchBrowser` option is `true`.

### Example:

```shell
# Override the browser launch command; use chrome's "--app"
#   argument to get a new, chromeless window for that "native feel"
ungit --launchBrowser=0 --launchCommand="chrome --app=%U"
```
better explanation in config.js
fixed case where neither launchCommand/Browser specified
Merge pull request #273 from namuol/master

Added custom launch command option.