feat: remove route in router

[SaltySooda]

feature： 'remove' is complex feature . This submit only gives the basic framework, and relevant details need to be improved later, such as modifying the priority of nodes

test：already make the basic test

ps： this is my first time to submit PR to gin，I have great passion to make contribute。please tell if I did not go the right way

[SaltySooda]

#776

[ashangpeng]

@appleboy please review this pr. thanks .

[thinkerou]

maybe we should separate one gin-contrib, https://github.com/gin-contrib/graceful

[SaltySooda]

@appleboy finish TODO 。

@thinkerou It is all middleware in gin-contrib. you think the "REMOVE" function should be there?