Feature/add unicode style

[tducret]

Hi there !

This PR adds a new style, called UNICODE.
Here is an example (and it is even nicer in the terminal).
I would love to see it in the next pip package :)

Thibault

┌───┬───┬───┐
│ a │ b │ c │
├───┼───┼───┤
│ 1 │ 2 │   │
├───┼───┼───┤
│ 3 │   │ 4 │
└───┴───┴───┘

[coveralls]

Coverage increased (+2.7%) to 80.613% when pulling 7a38ae0 on tducret:feature/add-unicode-style into bcfdb92 on kxxoling:master.