A number of fixes and maintenance changes.

Thanks to all contributors and reporters!

Changelog

• README: clarify search/show
• README: add install instructions
• replace string literals with constants
• remove redundant flags
• gnu uses different format than bsd and git grep
• linter error fix for regex
• fix #79, define a new greptype bsd to select : seperator
• use golangci-lint v1.26.0
• CI: use golang:1.14
• update dependencies
• build(deps): bump github.com/sirupsen/logrus from 1.5.0 to 1.6.0
• Update chmod command
• Cross-platform make install