Limit string length to provide color and link

[renkun-ken]

Close #313.

For link provider, the string length is limited to 255 characters.

For color provider, the string length is limited from 3 (shorted color name, e.g. red) to 20 (longest color name, e.g. lightgoldenrodyellow).

[randy3k]

perhaps it is worth to put down some comments.

[renkun-ken]

perhaps it is worth to put down some comments.

What do you mean by "put down some comments"?

[randy3k]

I meant to have some comments to explain the strange numbers like 256 or 20