Reuse of repeated parameters in config

[yurakuratov]

Sometimes duplicated parameters with equal values may appear in config file.

Look at the example below:

"pipe": [
{
"name": "squad_preprocessor",
"context_limit": 400,
"question_limit": 150,
"char_limit": 16,
},
{
"name": "squad_vocab_embedder",
"context_limit": "#squad_prepr.context_limit",
"question_limit": "#squad_prepr.question_limit",
"char_limit": "#squad_prepr.context_limit",
{
"name": "squad_vocab_embedder",
"level": "char",
"context_limit": "#squad_prepr.context_limit",
"question_limit": "#squad_prepr.question_limit",
"char_limit": "#squad_prepr.char_limit",
},
{
"name": "squad_model",
"id": "squad",
"word_emb": "#vocab_embedder.emb_mat",
"char_emb": "#char_vocab_embedder.emb_mat",
"context_limit": "#squad_prepr.context_limit",
"question_limit": "#squad_prepr.question_limit",
"char_limit": "#squad_prepr.char_limit",
}
]

It could be more convenient if there was no need to copy context_limit, question_limit, char_limit and to fill them from something like global parameters scope.

[yoptar]

This can also be used for paths with default values for configs, requirements and data

[yoptar]

Done in #574