structured/sectioned .driverc configuration

[odeke-em]

Fixes #778.

Now allows you to restructure your .driverc separating
commands into sections and having different clauses.

Please note:

• You can put multiple commands in the same
  section by using separator "/". See the rules at
  https://github.com/odeke-em/namespace/blob/0ab79ba44f1328b1ec75ea985ad5c338ba3d56a6/ns.go#L3-L14

• Overwriting of command rules is
  command_section > global
  and
  commandX_higher_line > commandX_lower_line
  where lines are natural numbers that are ascending

• Exhibits:
  This is a sample of a .driverc that I've just defined that has flexible
  behavior in different sections.

id=true # Will be applied globally unless overwritten in a command

[global]
depth=10 # The depth to use if not defined

[pull/list]
depth=2 # The depth to be used for pull or list

[push]
verbose=true # verbose is only true for push

[list]
long=true # Allow long output for list

And see the tests in src/rc_test.go that lock in the promised behavior.