fix: Added a shortcut for generating tag attributes (#73)

[Michael-F-Bryan]

• Added a modified version of the one in the issue
• Wrote some tests in a new test/element_test.py file
• Also added an example/doctest as part of the docstring for the _element_attributes() function

[Michael-F-Bryan]

I changed the example in #73 a bit so you are only given a sub-string that you can then add more complicated attributes to. For example:

class Footer(Element):
....

    def __str__(self):
        output = [ "<footer" ]

        attributes = [
            ('ident', 'id'),
            ('cl', 'class'),
            ('style', 'style'),
        ]
        sub_str = self._attrs_to_string(attributes)
        output.append(sub_str)

        if self.attrs:
            for k in self.attrs.keys():
                output.append(' ' + k + '="' + self.attrs[k] + '"')

        output.append(">")

        for child in self.children:
            output.append(str(child))

        output.append("</footer>")

        return "".join(output)

@tryexceptpass, It doesn't eliminate all the repetition but at least now you only need to handle the special cases. I imagine that a bit of refactoring might even iron out most of those as well.

[tryexceptpass]

Looks good, few things I'm thinking about before pulling in though:

• For a good number of items, attributes will just be cl and ident. May make sense to have attributes as a keyword arg with a default: attributes=[('cl','class'), ('ident', 'id')].
• I'd like it to handle attributes that don't have values, so if the attribute value is None, or maybe something else that makes sense (though not True or False), it just adds the word to the string. For example: if element has cl="abc", ident="123", checked=None, it would translate to id="123" class="abc" checked.

[Michael-F-Bryan]

I like the idea of adding that default argument. I've added a couple lines and a test to make sure it works.

I don't think I've ever seen/used a tag like <div id="123" class="abc" checked> before, but it was a fairly trivial thing to implement and test.

[tryexceptpass]

Pulling in... Thanks a lot for the help and suggestions.

On your point about the attributes by themselves, yes they seemed odd to me, but there are a few things in bootstrap that work that way.