Proper file isolation
While working on a3f02a6e, I noticed that ./bin/mdspell -r -a README.md
and ./bin/mdspell -r -a *.md
result in different found spelling errors before that commit.
This is due to how the spellchecker currently works, as it is initialized once, and not once per file. This means, that the file-specific excludes are applied globally and therefore the check results in different results for the same file, depending on whether it's checked alone or with other files that have individual added words in the .spelling
.