Some "improvements" but messy

[dionisos2]

Hi, I did some changes on my own repo if anyone is interested (on the "dionisos" branch) :

• Only correct either the lines around the point, or the visible part of the buffer, to avoid sending big requests to LanguageTool.
• Add a little cache system to avoid sending a request if nothing changed since the last correction.
• Make languagetool-correction-keys customizable.
• C-i save the rules to ignore in a global json file instead of the edited file (still local to each file)
• C-a save the words to ignore inside a global file (not local to each file anymore)
• Added the variable languagetool-core-correct-predicates to allow users to add their custom rules to ignore some words.
• Added languagetool-correct-buffer-forward to correct from the start instead of from the end.

My modifications are a little messy and break how some things worked before, this is why I don't propose a push request, but I share it here if anyone is interested in these changes.