Add a substring modification

[arieroos]

I sometimes get "yes" or "no" inputs, but I'm only interested in the first character ("y" or "n"). I think substring will be a handy addition, as it'll allow users of this library to discard or keep a certain amount of characters of their input.

I implemented substring with the following rules:
Tag syntax: substr:start[-end]

• start and end is 0-indexed
• If end is left unspecified, the substring will be taken from start to the end of the input.
• If neither values are specified, the tag does nothing.
• If start is greater than or equal to the length of the input, an empty string is returned.
• The same happens if start is greater than or equal to end.
• If end is greater than or equal to the length of the input, the substring is taken from start to the end of the input.

I think these rules are robust, intuitive, and simple for the client code to use.

[coveralls]

Coverage: 96.418% (+0.2%) from 96.246% when pulling e59c970 on arieroos:master into 0215b84 on go-playground:master.