Add leetcode-ignore-region to ignore lines.

[zhaozhixu]

Sometimes we may want to write some custom code, maybe for better code-completion support, which should be ignored during submissions. leetcode-ignore-region introduces 2 string variables which marks the begin/end of that region.

[zhaozhixu]

Quote from the README.md:

If you want to have some code before your solution, which may make auto-completion work better but shouldn't been submitted, you can customize leetcode-ignore-region to ignore lines in your code. For example, suppose you have a common.h and some custom definations, and leetcode-ignore-region is set to '("" "// start code"), only code below // start code will be submitted:

#include "common.h"
... your custom definations ...
// start code

class Solution {
    ......