Make it easier to start time monitoring for merge requests

Right now you have to write this:

```bash
gtt start --type=merge_request "kriskbx/example-project" 1
```
I was thinking about this:

```bash
gtt start -m "kriskbx/example-project" 1
```
... and for issues (if you configured the type in your config and want to override it):

```bash
gtt start -i "kriskbx/example-project" 1
```