Add hostname support for install/join

This release adds hostname support for the install and join commands as requested by the community.

Example with EC2:

export SERVER="ec2-3-250-131-77.eu-west-1.compute.amazonaws.com"
k3sup install --host $SERVER \
  --user ubuntu \
  --ssh-key $HOME/ec2-key.pem

export AGENT="ec2-3-250-131-78.eu-west-1.compute.amazonaws.com"
k3sup install --host $AGENT \
  --server-host $SERVER \
  --user ubuntu \
  --ssh-key $HOME/ec2-key.pem

Changelog for 0.9.12:

• PR #290 Add hostname support for install/join by @alexellis
• PR #283 github actions pinned to go 1.13 by @developer-guy
• PR #286 Add table of contents. by @yankeexe

Commits
7fb6fdf Update README and install/join help examples for hostnames by @alexellis
18b3601 Add hostname support for install/join by @alexellis
e7a216a Add table of contents. by @yankeexe
2a40410 Add table of contents. by @yankeexe
a10801b Rename to build by @alexellis
fed9fb3 Build all branches by @alexellis
b1fa224 Add OpenStack post by @alexellis
826a071 Disable hacktoberfest by @alexellis

Generated by Derek