Clarify handling of filtered terminals

Fix DeprecationWarning in lalr_analysis.py

As discussed in issue #378, when an embedded transformer (that is, one
passed to the Lark class using the transformer argument), is an
inplace transformer (either a subclass of Transformer_InPlace, or with
the @v_args(tree=True) decorator), the in-place transformer was not
working correctly and in-fact Lark used it like a normal non-in-place
transformer, expecting it to return the transformed value.

Allow any graph attribute in `pydot__tree_to_png`.

Keeping the explicit `rankdir="LR"` as default, add `kwargs`
to `pydot__tree_to_png` and `pydot.Dot` so that all graphviz
attributes are available for the graph.

Under python 3.3+, logging.warn is deprecated.
Use logging.warning instead.

Fixes: /Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/lark/parsers/lalr_analysis.py:87: DeprecationWarning: The 'warn' function is deprecated, use 'warning' instead

Bugfix/364 reconstruction update start symbol of parser instead of recreating it

load_grammar now collects all imports to make before loading them to …

Run tests against Python 3.7 and pypy 6.0

import_grammar now include base_path in recursive call to load_grammar

get_namespace_name now preserves leading underscore on relative import