genhtml: Implement option to specify coverage rate precision

Add command line support and config file support for specifying the
coverage rate precision, valid precision range: [1,4].

Signed-off-by: Euccas Chen <euchen@qti.qualcomm.com>
man: Add description for --precision and genhtml_precision

Add man page sections for genhtml's command-line option --precision
and lcovrc configuration setting genhtml_precision. Also add an
example configuration setting in lcovrc.

Signed-off-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>