Normalize accept HTML attribute value as a comma separated string

[UnknownPlatypus]

The html accept attribute expect a comma separated list of string.
This is easy to normalize the same way we normalize class as a space-separated list of string so I went for it.

This could possibly be expanded to other attribute with defined semantics (See #40 (comment)).

-<input type="file" accept=" image/gif
-                               , image/jpeg ">
+<input type="file" accept="image/gif, image/jpeg">

[g-plane]

I'm neutral on this, so I'm going to leave it open for discussions.

[UnknownPlatypus]

For context this is tracked more generally in prettier too Format values of attributes with defined semantics and led to the formatting of class and style attributes.

I've also done some research for other good candidates from the HTML attribute list (cross referencing the HTML spec) and found the followings that could be formatted space-separated like class:

• rel on "form", "a", "area", "link"
• autocomplete on "form", "input", "select", "textarea"
• sandbox on "iframe"
• accept-charset on "form"
• aria-labelledby, aria-describedby, aria-controls and aria-owns on any element

Quick searches using https://sourcegraph.com/search found matches with weird spaces for all of them so I suppose it could benefit some people.

[Hebilicious]

I personally love this as a default, but perhaps having an option to disable it would be useful for some users.