Add per-depth header highlighting and refactor highlight group configuration

[thecodewarrior]

1. Adds a new key in highlight_groups for header highlights
2. Extends the interpretation of highlight_groups values:
   • Allows specifying an existing highlight group by name
   • Expands the table to support specifying the full nvim_set_hl() value

This new highlight group interpretation includes one minor breaking change, which is that setting fg or bg to nil will no longer default to "NONE". In the past if both fg and bg were nil (the default state), the highlight group would simply be left uninitialized, so to maintain compatibility in this case the title, border, and text defaults have been changed to nil, which similarly falls back to an undefined group.

This implementation is currently dependent on #40, because as part of highlighting the header rows it needs to get the length of the indent, so it knows what column to start the highlighting. To do this is calls #get_indent(depth), which is a function defined in #40. If #40 is not merged this can be refactored to hard-code (depth - 1) * 2.

[AntonVanAssche]

Hi, #40 has been merged, please rebase your branch to include the indent option.

[AntonVanAssche]

Thanks!