add `start-end` mode

[julien-devatom]

Pull request

The mode is going to add the possibility to create addresses that start and end with a giver hex character, such as 0x1111....1111

Changes

• I have created a new mode parsing the start-end flag, with one hex char as argument
• I have also modifed a bit the kernel code to add a new score function for the startend mode
• I have updated the readme with the new mode in the "Modes with argument" section