Don't escape percent signs that are actually required in default mua command

I'm not sure why this was done originally, but it's only formatted once, so the escaped percent signs cause the formatting to simply replace the escaped ones with unescaped ones.

Closes: #59 (closed)

merged

mentioned in commit 20539d84

really not sure what happened here... it seems the default_mua variable was introduced in 55ee29fe with single % signs... but i can't pinpoint exactly when that was changed... it seems it was introduced when the argparse port was completed, but no rationale was given there.

it could be a relic of some tests i did where i would interpolate the string twice, but i can't find a trace of that, so let's drop it.