Adding support to customize the current window colors

[mrbrandao]

Those changes enables support to customize the current window foreground and background colors

• Enabling the support to customize the current window fg/bg colors via @dracula-current-window-colors
• Enabling as well support to customize the inactive window fg/bg colors via @dracula-window-colors

Such as:

set -g @dracula-current-window-colors '[background] [foregroud]' 

Examples:

# set the background of the current window status color to `dark_purple` and the foreground to `cyan`
set -g @dracula-current-window-colors 'dark_purple cyan'

# define the inactive window background to `gray` and foreground to `pink` 
set -g @dracula-window-colors 'gray pink'

Ps: This PR might have relation with the issue #128