Started framework for subcommand CLI demo app

Based largely on the idea here: https://gist.github.com/randomphrase/10801888
Intermediate commit.

Work on subcommit feature is not complete.

[ci skip]
readme: travis icon for branch subcommands

[skip ci]
Merge branch 'subcommands' of github.com:ulrikpedersen/swmr-testapp into subcommands
Intermediate commit
Turned SWMRWriter into a proper module

 * Removed main function
 * Moved class definition into a separate header file
 * Turned function args into const references where possible
Implemented the single application CLI with read and write subcommands
Travis: run the new single application
Added reporting function to SWMR reader
Added ability to set the write loop period

Added writer CLI option to set the period.
Tweaked logging output

Moved a bunch of debug messages to trace level.
Updated README.md to point to travis master branch