Tag cloud should limit the tags in fequency order instead of display order

[timnew]

When amount is set to tag cloud helper, the tags more than the amount won't be displayed.
But the issue is that the limit logic happens in display order.

But the common requirement is "As a blogger, I want to display the most used 40 tags in alphabetic order", which is impossible to achieve with current tagcloud helper.

[timnew]

Fixed with plugin hexo-helper-simple-tagcloud

[leesei]

@tommy351 what do you think if we change the default setting of tagcloud helper?

[leesei]

relates #507