Improve usability and/or readibility of help and apropos output.

The "help" command can output long list of command names or classes.
Use the title style to style the command names or classes to make
the output more readable.

Similarly, change "apropos" command to also style the command names.

It is sometimes unclear why "apropos REGEXP" lists some commands,
and then the user has to manually do 'help command' for all commands
listed by "apropos" to see more details about the matching commands.

=> Add an optional flag -v so that "apropos -v REGEXP":
  * outputs the full documentation of matching commands.
  * highlights the documentation parts matching REGEXP.