[Proposal] Add ipfix-elements.xml configuration file command-line argument

[mattijsjonker]

I propose to add an "-e" argument to ipfixcol, which allows one to specify the path to the ipfix-elements.xml file. This would work analogous to the "-c" and "-i" arguments for the startup and internalcfg files, respectively. It will come in handy if a user does not have write access to the default file (/etc/ipfixcol/ipfix-elements.cfg).

[thorgrin]

Added in d3a625cdd8f7732bcdcbd2131bb97ed42240145a