Drop support for Go 1.11
Use md2man v2 to avoid dependency issues when building with Go Modules
Enabled
Update TravisCI build environment images (trusty was deprecated)
Add optional Windows build
This adds two new methods to the `App` struct:
- `ToMarkdown`: creates a markdown documentation string
- `ToMan`: creates a man page string
Signed-off-by: Sascha Grunert <mail@saschagrunert.de>
