Image properties and onImageTap for the richTextParser, plus some fixes

[andreyeurope]

As we have talked, I have added an image properties attribute to the flutter_html library.
More than that, I have previously tried to use the 'alt' HTML element, but was not working.
I have fixed that.
I have also added onImageTap support for the richText parser.

Note: I don't know how you like your project structured, but I usually use a different file for a different class.

[andreyeurope]

@Sub6Resources Please take a look whenever you have time.
I have also other ideas for this library, like adding support for customStyle for headings and body. (I usually use the guidelines from Material Design)

[The-Redhat]

Since your are it. It would be nice if you add showImage as a Parameter. My
use case is to give the user the option to disable image loading without WiFi.

[andreyeurope]

@The-Redhat When this PR gets merged, you should be able to use the showImages parameter.
It works for both the richText and default parser.

[Sub6Resources]

Looks good. Thank you!

[w4mxl]

@andreyeurope @Sub6Resources

Can onImageTap Function consider adding image url or image data as parameters ?

This makes it easy to do some other processing when onTap, such as a large image preview.

Thanks！

[andreyeurope]

@w4mxl I will create a new PR for it.
I think it would be useful for me as well.
I will add an argument representing the source of the image ( so it will be a string containing the URL, the asset path etc.).

[w4mxl]

@andreyeurope Sounds good. Thank you!