A3T

- Added TERMINATED tag to all args following the terminator.
- Added braces for clarity.
Bug

- With FEATURE_STDIN defined, iostream is required.
A3t

- Added (disfunctional) A3t::append_stdin method.  Disfunctional in that
  it breaks a lot of tests because they use pipes, although it works.
- Added A3t::get_overrides to extract rc:<file>.
- Added A3t::get_data_location to extract rc.data.location:<location>.
Context

- Enabled initial A3t setup, migrating some functionality away from A3.
- Added table of attribute modifiers, for use as parsing entities.
- Context::initialize is now a hybrid of A3 and A3t processing, with the
  transition under way.
Code Cleanup

- Removed obsolete A3::rc_overrides and A3::get_data_location methods.
A3t

- Implemented A3t::apply_overrides.
Context

- Merged new and old code so that A3 and A3t calls are interleaved, making
  it easier to transfer processing.
Code Cleanup

- Removed redundant A3::apply_overrides call.
Code Cleanup

- Removed odd comment and unused member.
- Removed commented-out code.
Context

- Added helper, command and attribute entities.
A3t

- Added 'helper' entity support.
Context

- Now distinguishes between readcmd and writecmd entities.