Added normalizer for perldoc -f [built-in func]

[ypresto]

When a query only has lower-case characters, append '-f' option to let perldoc search for built-in functions.

• pros: Jump to built-in functions are done by K key
  • symlink
  • eval
  • open
  • etc.
• cons: Jump to pragmas should use :Ref command
  • use strict
  • use warnings
  • use constant
  • etc.