Add rulerfmt option for improved ruler customization

[joelim-work]

• Resolves Using `rulerfmt` to style the ruler #1358
• Fixes Show number of hidden items in the ruler #1287

Description

While the current ruler option can be used to configuring the contents of the ruler, it lacks the ability to customize styles and colors, and is also inconsistent with other UI options such as promptfmt and statfmt. This change builds upon the existing design of statfmt and applies it to the ruler, and encourages the deprecation of the existing ruler option.

Specification

• Placeholders for internal data
  • %a is the pressed keys
  • %p is the progress of file operations
  • %m is the number of files to be cut (moved)
  • %c is the number of files to be copied
  • %s is the number of selected files
  • %f is the filter
  • %i is the position of the cursor
  • %t is the number of files shown in the current directory
  • %h is the number of files hidden in the current directory
  • %d is the amount of free disk space remaining.
• Placeholders for options
  • %{lf_<name>} for the value of options (e.g. %{lf_selmode})
  • %{lf_user_<name>} for the value of user-defined options (e.g. %{lf_user_foo})
• Formatting for optional fields
  • Use | to divide the ruler into sections
  • Any section containing a failed placeholder substitution (blank result) is omitted

Examples

Display information about the cursor position:

set rulerfmt "%i/%t"

Display the number of cut/copied/selected files, only if there are any:

set rulerfmt "  cut: %m|  copy: %c|  select: %s"

Display the filter in blue with reversed colors:

set rulerfmt "\033[7;34m%f\033[0m"

Display the filter in blue with reversed colors and surrounded by spaces:

set rulerfmt "\033[7;34m %f \033[0m"

Display the value of the selmode option:

set rulerfmt "%{lf_selmode}"

Display the value of a user defined option:

set rulerfmt "%{lf_user_greet}"
set user_greet "Hello, World!"