Add excluded_filetypes

[yehuohan]

Add configuration excluded_filetypes to skip specified windows and buffers.

For example, we can setup hop.nvim with

require('hop').setup{
    excluded_filetypes = { 'scrollbar' }
}

to avoid crash when running HopChar1MW with plugin chentoast/marks.nvim enabled.

[yehuohan]

@phaazon I have updated the commit and rebased on the lastest commit. And this PR is ready for review now.

[Zeioth]

EDIT: To avoid hop from kicking in a single buffer, the solution described in #370 can be used.

This #309 PR works as intended, which is for multi window (different case of use from #370). We could merge it if you want. If @phaazon don't want to add extra complexity (as this is a kinda niche scenario), we could just keep the PR opened so interested people can use @yehuohan branch. This is an OK solution while both branches are still similar.

Up to you! Thank you.