fix #755 - macros: avoid KeyError for 'a' vs 'A'

Treat all register names as lowercase. At the moment, there is no
difference between 'a' and 'A', i. e., 'A' will not append
commands to the existing macro.
Merge pull request #756 from my-personal-forks/fix-755-macros-capital-letter

fix #755 - macros: avoid KeyError for 'a' vs 'A'