Add make install rule for easier testing

[bentsherman]

The current process for building and testing a plugin requires you to build Nextflow locally, include it in the plugin build, and use ./launch.sh to test the plugin with Nextflow in dev mode. This process is cumbersome and prone to errors,

An alternative which I first tried in the nf-prov plugin and is also being added to nf-validation here is to build and install the plugin directly to the plugins directory. While it may overwrite an existing plugin installation, it allows you to test the plugin without having to build Nextflow. It also avoids certain limitations of Nextflow dev mode (mainly that other non-core plugins cannot be used).