Interpolate values on Grammar creation

[ReubenJ]

Related to #69, it would be ideal to support something like the following use case when programmatically creating grammars.

n = 10
@cfgrammar begin
    A = |(1:$n)
end

or even something as simple as

n = 10
@cfgrammar begin
    A = $n
end

Currently, you have to make use of add_rule! to achieve something like this:

n = 10
[add_rule!(g, :(A = $i)) for i in 1:n]

or

n = 10
add_rule!(g, :(A = $n))

[janvandermeulen]

Design Issue

To solve this some variables will have to be substituted to get the desired behavior. But some languages naturally have variables. Take for example the following grammar

g = HerbGrammar.@csgrammar begin
    Real = |(0:9)
    Real = x
    Real = Real + Real
    Real = Real - Real
    Real = Real * Real
end

If x is defined in the context, should it automatically substitute it? In this case likely not, but how do you define the difference?