Add ClassNames util to attrs package

[chasefleming]

Closes: #165

[chasefleming]

@whisk what do you think about this? Do you mind reviewing?

[whisk]

hi @chasefleming,
I think that your point about missing spaces is valid. Concatenation of strings with + is error prone, and that's why I usually use something like

strings.Join([]string{"btn", elem.If(isPrimary, "primary", "")}, " ")

This is not exactly neat and may add redundant spaces, but it's safe.

Can ClassNames be generalized for other attrs which may need multiple values separated by spaces? I can't think of any right now.

Otherwise the PR looks good with one minor comment with no comments :)