A way to ignore unknown attributes #41

Hi there,

Cannot label it myself but this is a suggestion.

Building a custom tool based on MJML, I need to include to metadata inside this document.
My way of doing this for now is to include custom attributes in MJML nodes, based on this idea of a soft validation: https://documentation.mjml.io/#validating-mjml
As far as I understand this module, it's currently hardwired to be a strict validator, and rejects any custom attribute:

package main

import (
        "fmt"

        "github.com/preslavrachev/gomjml/mjml"
)

const m = `
<mjml>
 <mj-body>
   <mj-section>
     <mj-column>
       <mj-text custom-attr="some-value" font-family="Helvetica" color="#F45E43">
         <h1>Title</h1>
         <p>Paragraph</p>
         <p style="font-family:Comic Sans Ms">Another paragraph</p>
       </mj-text>
     </mj-column>
   </mj-section>
 </mj-body>
</mjml>
`

func main() {
        r, err := mjml.Render(m)
        if err != nil {
                panic(err)
        }
        fmt.Printf("Rendered:\n%.100s\n", r)
}

results in

panic: MJML compilation error:
        - Line 5 of (mj-text) - Invalid attribute 'custom-attr' for tag <mj-text>
…

I saw that one can add a hook on unknown attribute using

r, err := mjml.Render(m, func(ro *mjml.RenderOpts) {
    ro.InvalidAttributeReporter = func(tagName, attrName string, line int) {
        // do something
    }
})

but this cannot signal an acceptance.
I would have loved to put a return strings.HasPrefix(attrName, "custom-") in there to make it valid.

Is this a kind of feature you would accept?
If so, in what form? This is quite open as it can be this signature change just wrote (returning a bool), accept a whitelist somehow, implement global validation levels as shown in the first link, …