Resetting default GMT parameters - pygmt.config

**Description of the desired feature**

The `pygmt.config` class/context manager allows the user to permanently or temporarily override default GMT config parameters. If a parameter is set permanently (particularly in a notebook), it would be useful to be able to reset back to the default without having to reset the kernel / use `pygmt.config` as a context manager. Motivated by https://github.com/GenericMappingTools/pygmt/pull/482#issuecomment-647040633

**Are you willing to help implement and maintain this feature?** Yes

I've been able to override and reset everything myself by terminating and restarting the global session:

```python
from pygmt import session_management

session_management.end()
session_management.begin()
```

but this is obviously not ideal, and I fear this may have unintended consequences of which I'm ignorant. I see this feature more as something to facilitate figure _development_ in a notebook, but not so much for finalised figure scripts/notebooks, so it's perhaps not a huge issue?

Ideal usage would be something like:

```python
pygmt.config().reset()
```

I'm still familiarising myself with the inner workings, so I apologise if I've missed/misunderstood something important.