ENH: change display.width to None by default
#### Describe the solution you'd like

Currently  `display.width` is set to 80 char on a terminal as far as I understand, which often leads to output being truncated. It would be nicer if the terminal width was auto-detected, which would avoid repr truncation for some fraction of those cases. This can be achieved by setting `display.width=None` by default.

#### API breaking implications

This would likely be an issue for doctests. I imagine there should be a way to keep `display.width` inside a test session where doctests are checked. Though then it would mean that running some code in a REPL and in the doctest might not produce exactly the same output.

Overall, this would need more careful consideration but I'm wondering if there is any way to improve the usability for the interactive use-case.

#### Describe alternatives you've considered

Remembering which option needs to be set to avoid repr truncation and setting it. I keep forgetting which one it is, and have to spend a couple of minutes finding it again.
