Improvement of the recovery of Pragmatic Segmenter sentence segmentation text wrt to the original text offsets

[lfoppiano]

The pragmatic segmenter seems to modify the output string which makes the extraction of the sentence offsets more complicated.

This PR makes two changes:

• Implementation using a diff-based approach to extract correctly sentence segmentation offsets when the sentences are not matching 100%
• fix issue #753

Recovery of segmentation offset using Pragmatic Segmenter

This implementation uses an external library to compute the diff of the sentence and the original text.

For example

original = "This is the original text. Some spaces are going to be removed. sentence = "This is the original text."

The idea is to get the starting and ending within original taking as reference sentence. After translating the diff in a char based string, it compute the starting and ending of the sentence within the text.

The diff is a list of string where each element is structured as [operation, space, character], having operation = +, - or (e.g. comparing the string a and ab would result in the following: diff = [' a', '- b']

After the diff is computed using a two-pass heuristic:

• starting from left, we collect all the characters starting from the first character that is common a both strings
• starting from the right, we remove all the characters that do not equal in the diff.

The heuristic also is limited to a subset of the string, should a sentence has been identified before.

[coveralls]

coverage: 39.498% (-0.4%) from 39.903% when pulling 5aca6b85c306bafa2b94076ed36a745e64553b48 on sentence-segmentation-detection into 5b145364914d984ecbe6ec4afaaf57abd65b2a4a on master.

[lfoppiano]

The last commit a577523 should fix the issue #753