WIP better emp env/env-load

[ejholmes]

Closes #1097.

This is an attempt to make $ emp env | emp env-load better. Previously, emp env did not quote the environment variable values, which means multiline environment variables would show up like this:

$ emp set BAR=bar
$ emp set FOO="foo1
foo2
foo3"
$ emp env
FOO=foo1
foo2
foo3
BAR=bar

This isn't easily parsed by shells, and isn't supported in the .env format. After this change, emp env produces quoted values:

$ emp env
FOO="foo1\nfoo2\nfoo3"
BAR="bar"

Which means it can be piped into emp env-load or exported in your shell (e.g. export $(emp env).

This probably needs more test cases for various scenarios. Also, quoting is potentially backwards incompatible, so maybe it should be behind a flag on emp env.