Add command line arguments -diff_command and -multi_diff. (#495)

Flag -diff_command selects the diff command used for -mode=diff.  The boolean
flag -multi_diff indicates whether the diff command can compare more than two
files at once in the manner of tkdiff.

Update def.bzl to support the new command line arguments.
Use POSIX flags for the find command run in runner.bash.template.
Convert runner.bash.template to use xargs, so it will not fail when the number
of BUILD files is large.