add Binding.repl!, add Binding.repl#automatic!.

fixes #3
rename Binding.repl#automatic! as Binding.repl#auto.

ref #3.
remove 'options' argument. ref #3.

Binding.repl.auto could start (atm) one of three ruby consoles that
accept options that are understood differently by each console.

it makes passing 'options' error prone because when a different console
is loaded the options may not make sense anymore (and raise an error).
remove "binding.repl!", use "binding.repl.auto" instead. ref #3.

don't want to introduce another namespace on Binding(repl!), and
Binding#repl! expresses little intent on what it does.

binding.repl.auto retains the same namespace and expresses more
intent than binding.repl!.
re-implement "auto" to return exit value from REPL.

ref #3.
handle :'binding.repl.console_not_implemented', too.
fix bugs.
improve code, rename automatic_load_order as auto_load_error.
add no-op default proc.
Merge pull request #5 from robgleeson/feature/automatic-invoke-of-preferred-repl

add binding.repl.auto