Add tests and credit Bulbapedia in README

This should resolve #2.

test/test_pokemon.py

@@ -0,0 +1,37 @@
+import pokemon 
+import os
+import filecmp
+
+
+def test_download_national_index_html(tmp_path):
+    pokemon_html = tmp_path / "pokedex.html"
+    pokemon.download_national_index_html(pokemon_html)
+    assert os.path.getsize(pokemon_html) > 500000
+
+def test_get_pokemon_table_row_soups():
+    national_index = "test/test_pokedex.html"
+    row_soups = pokemon.get_pokemon_table_row_soups(national_index) 
+    assert len(row_soups) == 994
+
+def test_extract_pokemon_from_table_row(tmp_path):
+    national_index = "test/test_pokedex.html"
+    pokemon.POKEMON_CACHE_DIRECTORY = tmp_path
+    row_soups = pokemon.get_pokemon_table_row_soups(national_index) 
+    p = pokemon.extract_pokemon_from_table_row(row_soups[42])
+    assert p.name == 'Vulpix'
+    assert p.index == '#037'
+    assert p.html_url == 'https://bulbapedia.bulbagarden.net/wiki/Vulpix_(Pok%C3%A9mon)'
+    assert p.img_url == '//archives.bulbagarden.net/media/upload/thumb/3/35/037Vulpix-Alola.png/70px-037Vulpix-Alola.png'
+    assert p.img_filename.endswith('vulpix.png')
+    assert p.json_filename.endswith('vulpix.json')
+    assert p.description == ''
+    assert p.appears_in_book == False
+
+def test_extend_pokemon(tmp_path):
+    national_index = "test/test_pokedex.html"
+    row_soups = pokemon.get_pokemon_table_row_soups(national_index) 
+    p = pokemon.extract_pokemon_from_table_row(row_soups[42])
+    p.img_filename = tmp_path / 'vulpix.png'
+    pokemon.extend_pokemon(p)
+    assert filecmp.cmp(p.img_filename, 'test/test_vulpix.png')
+    assert p.description.startswith("Vulpix (Japanese: \u30ed\u30b3\u30f3 Rokon)")