Added cite command

Without true(), plunit does not check anything.
The purpose of this test is mainly to check command parsing, so
command/0 should be invoked.

initialize/0 can now be run several times (doing nothing except the
first time), without leading to a bogus state.

swipl "-q" option changed both the verbosity of the compilation and the
initial runtime verbose flag. This caused plunit not to print unit names
until the verbosity was restored to normal (e.g., by the
conservation_laws unit tests). I suppose that it was unattended.

Some minor fixes in coding style.