Now the plugin can be installed via tmux-plugins/tpm in addition to
being installed through zsh's antigen package manager.
It now only calls `make install` when installed through antigen; the
tmux plugin automatically compiles the binary, but leaves it in the
plugin directory.
This also allows skipping the "if exists" check, which means plugin
updates can also automatically rebuild any changes to the compiled part
of the plugin.
Updated the README to reflect this new install option.
This is part of Issue #43.
