Add formatDate function to Go template syntax

This makes it possible to use date and time in initial values like this:

```yaml
initialValue: "{{ formatDate .Now "2006" }}"
```

I want to use this to configure my BranchPrefix like this:

```yaml
git:
  branchPrefix: "ruudk/{{ formatDate .Now "2006/1" }}"
```

Author: ruudk

pkg/gui/services/custom_commands/handler_creator.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"strings"
 	"text/template"
+	"time"
 
 	"github.com/jesseduffield/gocui"
 	"github.com/jesseduffield/lazygit/pkg/config"
@@ -236,17 +237,22 @@ type CustomCommandObjects struct {
 	*SessionState
 	PromptResponses []string
 	Form            map[string]string
+	Now             time.Time
 }
 
 func (self *HandlerCreator) getResolveTemplateFn(form map[string]string, promptResponses []string, sessionState *SessionState) func(string) (string, error) {
 	objects := CustomCommandObjects{
 		SessionState:    sessionState,
 		PromptResponses: promptResponses,
 		Form:            form,
+		Now:             time.Now(),
 	}
 
 	funcs := template.FuncMap{
 		"quote": self.c.OS().Quote,
+		"formatDate": func(t time.Time, layout string) string {
+			return t.Format(layout)
+		},
 	}
 
 	return func(templateStr string) (string, error) { return utils.ResolveTemplate(templateStr, objects, funcs) }