This way it is more easily extendable.
This also gives the possibility to modify it in ftplugins. I thought
about removing 'de' from the dictionary in the tex.vim ftplugin to
encourage using `\"a` and alike.
`&spelllang` can be a list of multiple languages. Support that by
matching the language at the start, the end and/or between commas,
instead of comparing the value, and then appending to `l:ignore_chars`
for every matched language.
Apply the following:
1. Switch to `matchadd` as `match` can only be used for **one**
highlight
2. Use an autocommand group for better behaviour when reloading the
vimrc.
3. Use just one pattern as it seems to work always. (Apparently `\%#` is
not matched in normal mode)
