add spec of Plock.format
fix typo inspect => inspect_block

I forgot to change the description after renaming Plock.inspect into Plock.inspect_block
implement Plock.format with the dependencies
to avoid overhead in creating format strings call by call, constantize %b and %r
add spec to check if output_format descructively does not change the state of Plock
add spec to check if the altered p method prints out correctly
fix incorrect use of context
to make the spec more robust, add a specific before block to configure output format
considering portability, get rid of tap.
forgot to freeze DEFAULT_FORMAT
fix typo STDIN => STDOUT!!! OMG!!!
add format_with method to abstract format and pretty_format
now only alter pp when pp is defined
define print_block_with and modify p_with_plock and pp_with_plock with it