Support named conditional rules in dictionary files

[rhdunn]

The aim of this feature is to:

1. make the conditional rules easier to read;
2. allow the conditional rules to be more generic;
3. support more than the 9 conditional rules allowed by espeak (?1-?9).

Use Case 1: Accent Specific Rules

en_list dictionary file:

vase  v'A:z
vase  v'eIz  ?en-US

en-US language file:

rule en-US

Use Case 2: Accent Splits and Mergers

en_rules dictionary file:

ir  3:  ?bird-nurse # e.g. bird

en-GB language file:

rule bird-nurse term-nurse # FIR-FUR-FERN merger

In this example, the NURSE vowel (FIR-FUR-FERN merger) is treated as the special case. Without that, it will default to the rules for i and r, resulting in the correct pronunciation in Scottish English, etc..

Use Case 3: Yod

There are 4 types of yod (/j/) behaviour:

diphthong -- preserve archaic /Iu:/ clusters like 'yew' found in Welsh English, etc.
elision -- elide the diphthong (e.g. /nu/ for 'new' in American English).
coalescence -- /sj/ => /S/, /zj/ => /Z/, /dj/ => /dZ/ and /tj/ => /tS/ found in many varieties of English
preserving -- RP tends to preserve the yod sound in non-archaic clusters

These can occur in different environments in different accents. It should be possible to specify these behaviours in the pronunciation rules, then specify which behaviours apply in the language files.