Support global installation in our installer

[kylewillmon]

The install script currently only supports installing to the user's home directory (specifically, ~/.local/bin). Some environments would benefit from being able to install to a globally-accessible directory (probably /usr/local/bin).

Additional context

The need for this has come up as a part of phylum-dev/phylum-ci#118. The docker image created by the phylum-ci project installs the CLI into the home directory of the root user, which prevents the docker image from being run as a non-privileged user.

[kylewillmon]

Eventually, we will distribute phylum to the various Linux distributions with the appropriate package managers. Until then, we will leave the global install as a manual process.