(GNU options) - Implement -g, --chgexit

[ghostsquad]

Related to #4 this issue is to add additional options that make sense:

Options:
-b, --beep beep if command has a non-zero exit
-c, --color interpret ANSI color and style sequences
-d, --differences[=]
highlight changes between updates
-e, --errexit exit if command has a non-zero exit
-g, --chgexit exit when output from command changes
-n, --interval seconds to wait between updates
-p, --precise attempt run command in precise intervals
-t, --no-title turn off header
-x, --exec pass command to exec instead of "sh -c"

-h, --help display this help and exit
-v, --version output version information and exit

This issue is to implement the -g, --chgexit option