Enhancements (mostly documentation)

[BurdetteLamar]

This PR:

• Adds _doc/erb_executable.md: documentation for executable erb.
• Revises README.md, which now covers the executable erb as well as class ERB.
• Revises libexec/erb to improve the help text.
• Revises the documentation for class ERB.

A NOTE ABOUT TERMS:

• Formerly: The documentation used the term template to refer to an ERB object.
• In this PR: The documentation uses the term template to refer to the string input to ERB.new.

The reason for the change: When documenting the ERB executable erb,
we need a specific term that refers to its string input;
source is not a good idea, because ERB#src refers something entirely different;
the two different sorts of sources would bring confusion.

Therefore I've used the term template to refer to:

• The string input to ERB.new.
• The string input to executable erb.

I have not labelled this PR as [DOC] because it includes some changes to code.