Unexpected trim results

[ellioseven]

Getting an issue where sassdash will be too greedy when trimming characters like so:

$key: _trim("a--b--c--", "--");
@debug "Result: " + $key;

Results in:

DEBUG: Result: s

Which should be:

DEBUG: Result: a--b--c

[P233]

@davidkpiano The __chars-index function can not help when there is more than one character. In this case, I think it's better to split the $chars to a list, and iteratively slice the candidate character from the list on both sides of the string.

[davidkpiano]

Would be happy to accept a PR for this.