Remove limit on Abbreviate directive

[erkyrath]

This is the first half of #257. The MAX_ABBREV_LENGTH constant still exists, but it now only applies to the abbrevation generator. The Abbreviate directive can accept abbreviations of any length.

This required changing the abbreviations_at array (allocated in chunks of MAX_ABBREV_LENGTH) to a new abbreviations_text array (allocated in chars). The abbreviation struct now has a value saying where in abbreviations_text to look.