Parse espeak -X Flag for word to phoneme word mappings

[TediPapajorgji]

What does this PR do?
This PR introduces the new return_word_mappings parameter to the phonemize function to enable text to phoneme mappings for espeak backend only. In-turn when setting this value to True, appends the -X flag to espeak which prints out the mappings. See espeak PR here (coming soon).

What did you do?

• Added parameter return_word_mappings
• If return_word_mappings is true, append -X flag to espeak
• If return_word_mappings is true, parse -X flag output
• If return_word_mappings is true, return phonemized text and phoneme mappings, else just return phonemized text.

How did you test?

• Through the python console
• Modified resembletron's call to phonemize inside the synthesis.py#prepare_symbol_seqs_with_ssml function (added in Resembletrons PR #245)

What should the reviewer look out for?
Nothing.