Display current Tigris context in the shell prompt

Provide a command for displaying current Tigris context in the shell prompt, similar to completion:
```
tigris completion [bash|zsh|fish|powershell]
```

It can be: `tigris context shell >> ~/.bashrc`

and it could look like:
```
(tigris: my_proj1/my_branch1)$ _
```
also it can display an URL if it's not default:
```
(tigris: localhost:8081/my_proj1/my_branch1)$ _
```