Add a phapp environment command

[fago]

The command prints out the environment variables set after executing the bash command 'environment' from the manifest. It echos the variables that are new; thus for being able to do so it will have to compare the original environment with the modified environment and only print out variables that changed.

USAGE: $(phapp environment) custom-command

If --verbose is passed, it should also print out the executed command - as done by default by the /bin/bash prefix https://github.com/drunomics/phapp-cli/blob/master/src/Task/Exec.php#L68

The optional parameter "--show-command" makes phapp show the command instead of outputting the variables. Thus can be used as following:

eval $(phapp environment --show-command)

[fago]

could be probably done by running the "enviornment" commands from manifest via "env -i"
-> clean environent -> the enviornment commands adds the new stuff -> run "env" to output the environment

Then we should get only added environment variables on stdout

Related it might be nice to have
"phapp bash"
so you can use it to test "phapp.yml" scripts manually.