fix(envvar): use template for string value

docs/docs/segment-environment.md

@@ -50,7 +50,12 @@ The segment will show when the value of the environment variable isn't empty.
 ```
 
 - var_name: `string` - the name of the environment variable
+- template: `string` - A go [text/template][go-text-template] template extended with [sprig][sprig] utilizing the
+properties below. Defaults to the value of the environment variable.
 
 ## Template Properties
 
 - `.Value`: `string` - the value of the environment variable
+
+[go-text-template]: https://golang.org/pkg/text/template/
+[sprig]: https://masterminds.github.io/sprig/

src/segment_envar.go

@@ -18,7 +18,20 @@ func (e *envvar) enabled() bool {
 }
 
 func (e *envvar) string() string {
-	return e.Value
+	segmentTemplate := e.props.getString(SegmentTemplate, "")
+	if len(segmentTemplate) == 0 {
+		return e.Value
+	}
+	template := &textTemplate{
+		Template: segmentTemplate,
+		Context:  e,
+		Env:      e.env,
+	}
+	text, err := template.render()
+	if err != nil {
+		return err.Error()
+	}
+	return text
 }
 
 func (e *envvar) init(props *properties, env environmentInfo) {