Template Function Injection

This patch enables developers to add one to many template functions that
can be used by custom Usage and Help templates. Here is an example that
is included in the file cobra_test.go as the test function named
TestAddTemplateFunctions:

    AddTemplateFunc("t", func() bool { return true })
    AddTemplateFuncs(template.FuncMap{
        "f": func() bool { return false },
        "h": func() string { return "Hello," },
        "w": func() string { return "world." }})

    const usage = "Hello, world."

    c := &Command{}
    c.SetUsageTemplate(`{{if t}}{{h}}{{end}}{{if f}}{{h}}{{end}} {{w}}`)

    if us := c.UsageString(); us != usage {
        t.Errorf("c.UsageString() != \"%s\", is \"%s\"", usage, us)
    }

In the above example four functions are added to the template function
map used when the Usage and Help text is generated from the templates
that enable custom logic as well as data injection during template
execution.

cobra_test.go

@@ -8,6 +8,7 @@ import (
 	"runtime"
 	"strings"
 	"testing"
+	"text/template"
 
 	"github.com/spf13/pflag"
 )
@@ -971,3 +972,20 @@ func TestFlagOnPflagCommandLine(t *testing.T) {
 
 	checkResultContains(t, r, flagName)
 }
+
+func TestAddTemplateFunctions(t *testing.T) {
+	AddTemplateFunc("t", func() bool { return true })
+	AddTemplateFuncs(template.FuncMap{
+		"f": func() bool { return false }, 
+		"h": func() string { return "Hello," }, 
+		"w": func() string { return "world." }})
+
+	const usage = "Hello, world."
+	
+	c := &Command{}
+	c.SetUsageTemplate(`{{if t}}{{h}}{{end}}{{if f}}{{h}}{{end}} {{w}}`)
+	
+	if us := c.UsageString(); us != usage {
+		t.Errorf("c.UsageString() != \"%s\", is \"%s\"", usage, us)
+	}
+}