Improve Custom Template with Generics definition and generics where clauses

[bard83]

Introduce two specific sections which covers generics:

• genericsDefinition: defines the keys used inside the generic. The keys will be specified inside a string separated by commas.
  genericsDefinition: "I,J,K". The keys should be enclosed automatically between < and >.
• genericsWhereClauses: defines one or more generic where clauses. It's an array of string which the constraint of the related generic key. genericsWhereClauses: ["where I : class", "where J : struct", "where K : IMyInterface"]. The clauses will be joined by EOL and applied to the custom template.