Adding ability to define Functions

[rmcraig1978]

Using the configuration object to contains dynamic Function definition.

[jochenberger]

See also #286 for an alternate solution

[ghost]

Nice Feature ,
you should take care about something there is bug into FilterCompiler.class.
into this function
private boolean currentCharIsClosingFunctionBracket(int lowerBound){
if(filter.currentChar() != CLOSE_PARENTHESIS){
return false;
}
int idx = filter.indexOfPreviousSignificantChar();
if(idx == -1 || filter.charAt(idx) != OPEN_PARENTHESIS){
return false;
}
idx--;
while(filter.inBounds(idx) && idx > lowerBound){
if(filter.charAt(idx) == PERIOD){
return true;
}
idx--;
}
return false;
}

if you use function with argument into filter the token ')' will be miss
for example $.store[?(@.book.yourFunction(1))] will compile @.book.yourFunction(1
end bracket is missing because this condition will match ---> if(idx == -1 || filter.charAt(idx) != OPEN_PARENTHESIS

[cristph]

why gave up this PR halfway?
it's a very nice feature