Support the += operator in variable assignment.

[ShalokShalom]

Hi 🙂

I just tried to compile this code: https://github.com/CyberShadow/aconfmgr/blob/master/src/main.bash#L117

Error message:

main.sh(117:19): syntax error: expected `)`, found `--`.

[yassinebenaid]

Yeah, because bunster still doesn't support the += operator in variable assignment.

[ShalokShalom]

I changed the title of the issue accordingly. 🙂

[rayjonesjay]

I'd like to implement += operator