Improve release process & other fixes

[markdboyd]

Changes proposed in this pull request:

• add script for generating release binaries
• update installation instructions
• increase plugin version to 1.1.2 in metadata because I forgot to increase the plugin version with the last release
• remove vendor dir in favor of go modules

Things to check

• For any logging statements, is there any chance that they could be logging sensitive data?
• Are log statements using a logging library with a logging level set? Setting a logging level means that log statements "below" that level will not be written to the output. For example, if the logging level is set to INFO and debugging statements are written with log.debug or similar, then they won't be written to the otput, which can prevent unintentional leaks of sensitive data.

Security considerations

None