Proposed enhancements for issue #7, based on work by MSpekkio

[bwakabats]

Resolves issue #7 with the ability to register external function.
Based on work by @MSpekkio in #9

The static Parser.RegisterFunction allows the registration of a type (derived from IFunction) or an instance of the same.

The functions can:

• Add new functions (demonstrated in the Unit Tests for "Custom_Function_Test > JOIN"
• Extend existing functions (demonstrated in "Custom_Function_Test > MID")
• Replace existing functions (demonstrated in "Custom_Function_Replacement_Test > ISBLANK")

NOTE: There are 2 changes to the underlying code:

1. The name of the function is now derived from the FunctionDef.Name, rather than part of the IFunction class name. So a single IFunction could, in theory, accommodate more than one math function.
2. IFunctions must now be stateless (all existing internal IFunctions already are) since a single instance of each function is reused.

[jramoseguinoa]

@bwakabats nice work! @butulia @matheval can you please accept this PR and release a new nuget version?

[mikhail-teslya]

A wanted feature, is the one expected to be reviewed @butulia @matheval? Thanks for the effort :)

[dei79]

@mikhail-teslya, @bwakabats and @jramoseguinoa I derived my own version of this awesome library and included the external functions capability. Just case you need this feature: https://github.com/dei79/ExpressionEvaluationRuntime