Add support for combining pathspec objects via addition operator

[nhhollander]

This change allows for a set of rules to be expanded using the + and += operators.

I wrote these changes for a project of mine which collects patterns from multiple places and needs to combine them into a single PathSpec object, but for structural reasons can't practically combine the rules into a single list and pass to PathSpec.from_lines.

I also added and verified a couple of test cases for these new operators.

Example usage of + operator:

# Load first spec from a string array
spec1 = pathspec.PathSpec.from_lines('gitwildmatch', [
    'plans_for_world_domination.txt',
    'secret_family_recipe.txt'
])
# Load second spec from a file
fh = open('patterns.txt','r')
spec2 = pathspec.PathSpec.from_lines('gitwildmatch', fh)
# Combine the specs 
combined_spec = spec1 + spec2

Example usage of the += operator:

# Load first spec from a string array
spec = pathspec.PathSpec.from_lines('gitwildmatch', [
    'plans_for_world_domination.txt',
    'secret_family_recipe.txt'
])
# Load second spec from a file
fh = open('patterns.txt','r')
spec += pathspec.PathSpec.from_lines('gitwildmatch', fh)

[cpburnz]

The code and tests look good. Thanks for submitting the addition.