Support for IPA remappings and better maintainability

[ginic]

• Unifies conversion between all phonecode pairs into a single function and adds Phonecodes enum to enforce mapping behaviors. The x2y conversion functions now all call convert.
• Adds support for optional post-processing after conversion to/from IPA is performed, to allow for reduction to a shared symbol set. This is useful, for example, to convert standard TIMIT symbol reductions or a shared symbol set between Buckeye and TIMIT.
• Fixes ARPABET to IPA conversion places stress marks on wrong vowels when stress marker appears after vowel in token list #15
• Bump to version 2.0.0, since the underlying conversion code has changed significantly.