Allow add to accept package name and version as a single string #717

• I have searched the issues of this repo and believe that this is not a duplicate.

Issue

Please allow add to accept the package name and version constraints in a single string. I.e. poetry add 'something>=2.1' instead of having to use poetry add 'something >=2.1'.

This is slightly related to #429 in that it would also reduce friction for people switching over from other tools (e.g. pip).

Also there is a slight documentation bug here: Neither the docs for the add command nor the command line help include any mention on how to specify version constraints in the first place.