Change `foldtext` option only for current buffer instead of globally

[heygent]

Right now, when loading the foldtext module the foldtext option is changed globally. This overrides the value chosen by the user in all files and leads to inconsistent behaviors, like:

• you open file A, the foldtext option is what you set on your personal configuration
• you open file B, a Markdown file; foldtext gets changed and uses the Mkdnflow version
• you go back to file A, the foldtext now is what Mkdnflow set

This is fixed by simply setting foldtext using opt_local instead of opt.

[jakewvincent]

Hi @heygent, thanks a bunch for the fix; this was a silly mistake on my part. Would you mind updating the PR so the commit message is prefixed with a fix: ... tag? I'll merge the fix later today once that is done.

[heygent]

No problem, done. Thank you