Better parameter substitution support

[darkvariantdivine]

Presently, gameta supports the following methods of parameter substitution: parameter, constants and environment variables. This level of functionality is insufficient for more complex build chains which may require things like bash substitutions for parameters.

This feature proposes a unifying infrastructure based on regex that extracts, parses and outputs the intended parameter value. This infrastructure should support the following methods of substitution:

1. repository parameter
2. Gameta constants
3. multiple bash commands
4. or logic in substitutions

Parameter substitution should be parsed before command execution and apply to the following:

1. Gameta repository parameters
2. Gameta scripts
3. Gameta constants