Add alignment and fix a bug

[kuangkzh]

1. Fix bug: The previous levenshtein_distance return 0 when either input is "". Like levenshtein_distance("aaa", "")=0 which should be 3 instead.

2. Add function:

• lcs_sequence_idx

• lcs_string_idx

• levenshtein_distance_idx

• edit_distance_idx mapping indices from str1 to reference

• levenshtein_distance(str1, str2, weight) for weighted edit distance. All functions are updated with README and unittests.

3. Format code and doc:

• Longest common subsequence as lcs_sequence_xxx
• Longest common substring as lcs_string_xxx _xxx including _length, _idx, _of_list. For compatibility, all previous function names are all preserved.