Show variables that start with an underscore in the var view

We still filter things that start and end with __, to filter out things like
__builtins__, __file__, and so on. It also removes the filtering of variables
that start with ., which is only possible if something puts that in globals()
manually.

Fixes #292.

Improve some text width calculations

Show length of array types in var view

The new code shows length of set, frozenset, list, tuple and dict type
varibles in variable view.

set socket REUSEADDR opt for the remote debugger

update Go-to-Line UI with File/Current Line/Go to Line

check 'safely_stringify_for_pudb' on the type #276

The hasattr call should check for 'safely_stringify_for_pudb' on the type of the object
instead of the object itself, so as not to implicitly call its __getattr__ (if implemented).
This will avoid the exceptions on such magic objects.

Fixes the issue in the comment
https://github.com/inducer/pudb/issues/271#issuecomment-326776945. I don't
know if it also fixes the issue itself.

I am unsure if it really needs to be used for the variable name itself (can
variable names be double-width characters?), but I suppose it can't hurt.

Don't return early when "returning" from a module

We still need to run interaction so that the debugger will show the result of
the last line that was run. We just don't want it to treat the end of the
module as a "return".

For instance, if you have a file with a single variable definition

    a = 1

then if you stop on the definition, pudb will show the variable "a: 1" in the
variables view (but it won't show "return: None" after stepping again).

Fixes https://github.com/inducer/pudb/pull/264#issuecomment-326777042.

Check whether output from obj.safely_stringify_for_pudb() is actually a string, so as to not be confused by Mock objects (closes #270 on github)

Fix keyword arg error

fix lint errors

Add ptipython

Don't show "Return: None" when "returning" from a module frame

That is, when the last line of a module file is executed (usually from
runscript).

Use urwid's calc_width in make_canvas

This does the right thing for double-width and zero-width Unicode characters.

Fixes #232.

Tell ptpython to read/process the user's config file.