Modifiers and more treatment for <img> and <a> tags

In this PR some more handling is provided, to improve emails.

- Control over sizes of images: max size of 300 px is implemented (this can be made as setting in future). This is because if in html code for a plugin we have big images, then they are big in emails also. Now we set the limit.

- Convert ```<a> ``` tag to simple links (in text emails only).