Command line option to open new tab and to set title text of the tab

**Feature request:**

Allow opening new tabs and setting title from command line launch of cosmic-term. For example,

```
cosmic-term --tab
```

to open a new tab on current terminal window

```
cosmic-term --tab --title "Cool terminal"
```

to open a new tab on current terminal window and set its title (that shows up on the tab title bar)