Abbreviate context string so prompt doesn't overflow
Abbreviate context string so prompt doesn't overflow

When `cd`ing into an object or using `binding.pry` to trigger a
debugging session, the context string can overflow the prompt.

Example:

```
2.7.0 (jazz_fingers)[4] » Class1::Class2::Class3::Class4::Class5::Class6::Class7::Class8::Class9::Class10.new.speak
2.7.0 (#<Class1::Class2::Class3::Class4::Class5::Class6::Class7::Class8::Class9::Class10>)[1] »
```

This patch abbreviates the context string to display only its tail.

Result:

```
2.7.0 (jazz_fingers)[3] » Class1::Class2::Class3::Class4::Class5::Class6::Class7::Class8::Class9::Class10.new.speak
2.7.0 (#<...::Class7::Class8::Class9::Class10>)[1] »
```

aried3r/jazz_fingers,plribeiro3000/jazz_fingers