Add escape string helper

[ondrejnebesky]

I could use new handlebars filter to escape any string, so I can use it in JSON templates. Currently, some inputs can break JSON formatting in the template. The following characters should be escaped:

• Backspace to be replaced with \b
• Form feed to be replaced with \f
• Newline to be replaced with \n
• Carriage return to be replaced with \r
• Tab to be replaced with \t
• Double quote to be replaced with "
• Backslash to be replaced with \

Example

template:

{
  "body": "{{body 'Body'}}",
}

input: Body=Hello "World"
current output:

{
  "body": "Hello "World"",
}

expected output:

{
  "body": "Hello \"World\"",
}