Add customizable per-input-mode default text/background coloring for indicator statusline

[christianparpart]

We can currently only configure the default text/background color for the indicator statusline (active window and inactive window).

The goal is to make the color adapting based on input mode.

Possible configuration could look like:

profiles:
    your_profile_name:
        indicator_statusline:
            # default foreground/background for any non-override specified status line.
            foreground: '#808080'
            background: '#000000'
            # specific colors for the different modes
            overrides:
                # override default colors for e.g. insert_mode
                # possible also: normal_mode, visual_mode(any visual),
                #         visual_block_mode, visual_line_mode, search_mode(?)
                insert_mode:
                    foreground: '#123456'
                    background: '#abcdef'

Checklist

• [x] override for insert mode
• [x] override for normal mode
• [x] override for visual mode (any visual mode)
• [ ] override for visual line mode
• [ ] override for visual block mode
• [x] documentation added