Docgen: Introduce mechanism to define verbosity of output

[cepsdev]

We need a general mechanism to control the verbosity of the output written by document modules. e.g. console output triggered by the --pr option.

An interesting approach would be to introduce doc-templates.

Example:

Consider the following state machine definition.

sm{
S;
states{A;B;C;};
t{A;B;};
};

A user might be interested only in the name of the states and not the specific transitions. i.e. he would like a summary in the form:

State Machine S
States:
A, B, C

To achieve this, the user could define a docgen-template:

kind docgen_template; docgen_template DT;
template{
DT;
sm{
states;
};
};

Meaning: Of all top level elements print only sm - elements, and of sm -elements print only states.