Don't install several dependencies by default

[gunthercox]

Right now the issue tracker is prolific with tickets that have titles similar to "Help, I had an error while installing ChatterBot". The intention of this pull request is to make changes that eliminate a number of dependencies that could be considered optional from the primary installation process. Although I think spacy and chatterbot_corpus are the primary culprits; spacy requiring build dependencies to compile, and chatterbot_coupus currently installing via a GitHub url... I think the the idea of having dependencies required in order to enable features might be an advantage in general to reduce the footprint and efficiency of the overall ChatterBot installation.

[nbthinh]

In my code:
from chatterbot import ChatBot
from chatterbot.trainers import ListTrainer

Create a new chat bot named Transporter

chatbot = ChatBot("Transporter")

Create a new trainer for the chatbot

chatbot.set_trainer(ListTrainer)

conversation = [
"Hello",
"Hi there!",
"How are you doing?",
"I'm doing great.",
"That is good to hear",
"Thank you.",
"You're welcome."
]

Training

chatbot.train(conversation)

Get a response to the input text 'Good morning!'

response = chatbot.get_response("Good morning!")

print(response)
Have error: AttributeError: 'ChatBot' object has no attribute 'set_trainer'
Would you mind helping me to solve this problem? Thank you.