log merge commands to file
do not clutter output with long list of missing histos; just print how many
add warning for non-standard tag
isMonthDayTag
forgot to import isMonthDayTag
setOuputDir
cmd-line output directory
output dir also for FakePlotting::MCFakeRate

Details:
Matt says MCFakeRate and DataMCSF are the only two functions actually
being used.
cleanup MCFakeRate, DataMCSF (comment lines, empty lines, whitespaces, indentation)
cmd-line outputfile, used in DataMCSF
factorize mkdirIfNeeded -> dirExists + mkdirIfNeeded
cmd-line tag and inputdir
chmod +x
init FakePlotting after setting input/output/tag
outputfile is in fact InputItercorrFile; rename
add example in usage
cleanup plotSF: vertical compression, obsolete comments, indentation