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
These two dependencies are not really needed, which can be
reproduced via:
```
> export GO111MODULE=on && go mod tidy
```
Signed-off-by: Sascha Grunert <sgrunert@suse.com>
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>
