Add -indicator variables to customize the mode line

[DivineDominion]

To give users the ability to change "c" and "i" to something different.

I found the doom-modeline colored mode bullets more useful to see the currently active mode, maybe because the characters are rather small (with my config).

Users can now customize the indicators including whitespace like so (random example):

(setq xah-fly-insert-mode-indicator "● ")
(setq xah-fly-command-mode-indicator "– ")

[xahlee]

thanks.