lintian-junit-report: Allow passing options to lintian

This patch adds support for passing options directly to lintian,
allowing things like

lintian-junit-report \
    --lintian-opt=-Xsource-copyright,copyright-file *.deb

for those who like to break Debian Policy on purpose :). To be sure that
arguments are passed correctly lintian process is now executed with
IO::popen.
lintian-junit-report: Accept multiple file arguments

Usage note promises possibility of passing multiple files to check,
which hasn't been actually supported until now.