session: Add support for input format options (-I cmdline parameter)

The previous implementation supported the selection of an input format
by means of the -I command line option. This commit extends the feature
by adding support for colon separated input format options similar to
sigrok-cli.

This allows users to open files from the command line which previously
became only available after filling in dialogs, and resulted in errors
in the absence of options. Here is an example of how to use the option:

  $ pulseview -I csv:header:first-channel=2 -i filename.csv

This fixes bug #951.