Add runCommand function to Go template syntax

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

```yaml
initialValue: 'ruudk/{{ runCommand "date +\"%Y/%-m\"" }}/'
```

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

```yaml
git:
  branchPrefix: 'ruudk/{{ runCommand "date +\"%Y/%-m\"" }}/'
```

pkg/commands/git_commands/custom.go

@@ -1,6 +1,11 @@
 package git_commands
 
-import "github.com/mgutz/str"
+import (
+	"fmt"
+	"strings"
+
+	"github.com/mgutz/str"
+)
 
 type CustomCommands struct {
 	*GitCommon
@@ -18,3 +23,18 @@ func NewCustomCommands(gitCommon *GitCommon) *CustomCommands {
 func (self *CustomCommands) RunWithOutput(cmdStr string) (string, error) {
 	return self.cmd.New(str.ToArgv(cmdStr)).RunWithOutput()
 }
+
+// A function that can be used as a "runCommand" entry in the template.FuncMap of templates.
+func (self *CustomCommands) TemplateFunctionRunCommand(cmdStr string) (string, error) {
+	output, err := self.RunWithOutput(cmdStr)
+	if err != nil {
+		return "", err
+	}
+	output = strings.TrimRight(output, "\r\n")
+
+	if strings.Contains(output, "\r\n") {
+		return "", fmt.Errorf("command output contains newlines: %s", output)
+	}
+
+	return output, nil
+}