Support multiple override appends

[amitschang]

Currently only the last override supplied is considered, in conflict with now supported append overrides ("+="). This now enables appending multiple times via the command line.

The docs are also misleading for dependencies, where seems to imply a comma-separated list of dependencies can be given in the override, but there is no handling of this (and there probably shouldn't be, since , is a possible character in version spec).

With this change, something like

tox -e env -x testenv.deps+=debugpy -x testenv.deps+=pytest

would be possible.

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

[gaborbernat]

Changelog please

[amitschang]

@gaborbernat, done, thanks!