Added simple_square_brackets and expand_vowels transforms

[morwoen]

Hi, I was tasked with finding a pseudolocalisation library in Python and your project stuck out. Wanted to contribute back some of the extensions we've implemented :)

We've added two new transform functions:

• simple_square_brackets - add [ and ] around the string to signify the boundaries. Added to allow for more options for boundary characters.
• expand_vowels - implements the suggestion from the Netflix article https://netflixtechblog.com/pseudo-localization-netflix-12fff76fbcbe Keeps the string easier to read and understand and clear what is part of it and what isn't while still expanding the length.

In terms of backwards compatibility, there should be no issue. Each transform now receives the placeholder/format regex used in order to allow for more complex transforms (such as the expand_vowels which needed to know where all the placeholders are.

[morwoen]

Made a small change to the __get_target_length function - if a string contained only placeholders and we discard placeholders in length calculation the size will be 0. This ensures we return 0 as well to account for that.

[robaw]

Hi @leonidessaguisagjr , would you have interest in accepting this contribution. We'd like to use your tool (and contribute back).