Allow for appending to [a-z] registers

[DevinLeamy]

Is your feature request related to a problem? Please describe.

• Current I cannot append to [a-z] registers
• vim provides: "q{0-9a-zA-Z"} Record typed characters into register {0-9a-zA-Z"} (uppercase to append)"

Describe the solution you'd like

• Ideally, I could append to my [a-z] registers, like in vim

Describe alternatives you've considered

• I saw the de-facto vim implementation of this feature on the roadmap. I did not consider alternative solutions.

Additional notes

• I've implemented the desired changes on a fork. If this issue is given the OK, I'll submit a PR.