Allow to specify attributes without value in selector for html filtering rules

[anyakushin]

There are two cases for this:

• removing elements by matching attributes without values
• removing elements by matching attributes by name only, no matter what their values are

So the rule:

example.org$$script[defer]

should remove the element:

<script defer data-src="url"></script>

and the rule:

example.org$$div[some_attribute]

should remove both elements:

<div some_attribute="some_value">...</div>
<div some_attribute="another_value">...</div>

Related CoreLibs task: AdguardTeam/CoreLibs#1934

[anyakushin]

Also, do not forget to update versions that supports the new syntax in relevant section in Knowledge Base after implementing this.