WIP: add -1/--singleshot option

[r00t-]

WIP: fix #389

[r00t-]

@mbehr1:
maybe you can help, it seems you wrote most of the unit-tests:
my changes add a dependency on the options global to MeterS0.cpp,
(so that if if single-shot mode is enabled, MeterS0 can generate a semi-useful "power" reading after two pulses, instead of a useless single "impulse" reading.)
but this breaks the unit-test, because mock_MeterS0 links MeterS0.cpp without also linking Config_Options.cpp.
options should probably be replaced with a mock for that purpose, but it's not so straightforard for me,
can you provide at least a pointer to how you would handle this?

(alternatively one could write code to feed the single-shot flag into MeterS0 when constructing/configuring it,
but adding such meter-specific code to the nice generic meter-interface
doesn't seem better to me than accessing the global from the meter.)