Add option to explain lines

[roskakori]

Background

Sometimes the numbers pygount produces are not obvious, and users would like to know how a certain line is counted. Sometimes even developers (see #171).

Goals

• When the command line option --explain is specified, pygount prints each analyzed line and how it did count it.

Example output

For the source code

"""Test"""
print("Hello World")

The output would be something like:

d 1 """Test"""
c 2 print("Hello World")

with c=code, d=documentation, and 1 resp. 2 being line numbers.