Add markdown and man page docs generation methods

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>

flag-gen/templates/cli_flags_generated.gotpl

@@ -4,6 +4,7 @@ package {{ .PackageName }}
 
 import (
 	"flag"
+	"fmt"
 	"strconv"
 	"time"
 )
@@ -40,6 +41,22 @@ func (f {{ $flag.Name }}Flag) IsRequired() bool {
 	return f.Required
 }
 
+// TakesValue returns true of the flag takes a value, otherwise false
+func (f {{ $flag.Name }}Flag) TakesValue() bool {
+	return {{ $flag.Value }}
+}
+
+// GetUsage returns the usage string for the flag
+func (f {{ $flag.Name }}Flag) GetUsage() string {
+	return f.Usage
+}
+
+// GetValue returns the flags value as string representation and an empty
+// string if the flag takes no value at all.
+func (f {{ $flag.Name }}Flag) GetValue() string {
+	{{ $flag.ValueString }}
+}
+
 // {{ $flag.Name }} looks up the value of a local {{ $flag.Name }}Flag, returns
 // {{ $flag.ContextDefault }} if not found
 func (c *Context) {{ $flag.Name }}(name string) {{ if ne .ContextType "" }}{{ $flag.ContextType }}{{ else }}{{ $flag.Type }}{{- end }} {
@@ -66,4 +83,4 @@ func lookup{{ $flag.Name }}(name string, set *flag.FlagSet) {{ if ne .ContextTyp
 	}
 	return {{ $flag.ContextDefault }}
 }
-{{ end }}
+{{ end }}