Evaluate Highlight functions together with providers

[ben-hansske]

• Closes highlight function only called on init? #188.

• If the user passes a function or a table containing a function to highlight or separator_highlight, the highlight groups do get initialized, but do not get updated frequently.

• The solution would include having a second table, containing all the "dynamic" highlights, i.e. the ones that are/contain functions and need an update.

• If that list is not empty, add to vim.wo.statusline an luaeval, that updates the highlights.