Running `bin/install` doesn't handle `Plug` not being already installed

[pcapel]

Expectation:

Given I am a person who wants to install dotmatrix
When I run the bin/install script after cloning the repo
Then I should be able to run vim and have Plug installed

Actual Result:

Plug is neither installed or checked for, so the installation of dotmatrix causes vim to spit out a ton of errors related to the missing commands.

We could either run the curl command automatically in the setup script, or we could have the script warn that Plug is missing. Personally I think that installing it is nicest, but it will need to check for the file presence regardless of which path we decide on so I'm open to suggestions otherwise.

[pcapel]

I plan to work on the installation and flow for dotmatrix and other workstation related projects during open source time tomorrow (1-31-20), so any feedback is welcome prior to me creating a PR.