Improve random_rotate

[nattochaduke]

For now, chainercv.transforms.random_rotate rotates image by right angle (one of 0, 90, 180, 270 degrees).
However I believe that it is convenient that the degree of rotation is sampled from more fine distribution {i | -180 <= i <= 180} and the range of distribution can be specified, like pytorch's random_rotate.
So I propose to add arguments that enables this feature to random_rotate or to add another function to chainercv.transforms.

[yuyu2172]

Thanks for your suggestion.
I see both the current implementation and what you proposed convenient, so I should probably have two functions for them.
Thinking about it, I should have probably named the function as random_rot90.

[knorth55]

this is implemented in #690