allow hostnames to be specified via environment variables

[bryonjacob]

This PR allows for directing this SDK to a different set of URLs from the default set used at https://data.world

[laconc]

This is great. Perhaps, we could also add a snippet concerning this to the readme?

[laconc]

Love what you're doing here, @kndungu!

I did notice some flake8 issues that are preventing the checks from passing, specifically:

./datadotworld/files.py:31:1: E302 expected 2 blank lines, found 1
./datadotworld/client/api.py:41:1: E302 expected 2 blank lines, found 1
./datadotworld/hosts/__init__.py:28:1: E302 expected 2 blank lines, found 1
./datadotworld/hosts/__init__.py:29:3: E111 indentation is not a multiple of four
./datadotworld/hosts/__init__.py:30:3: E111 indentation is not a multiple of four
./datadotworld/hosts/__init__.py:31:3: E111 indentation is not a multiple of four
./datadotworld/hosts/__init__.py:34:3: E111 indentation is not a multiple of four
./datadotworld/hosts/__init__.py:36:1: E305 expected 2 blank lines after class or function definition, found 1
./datadotworld/hosts/__init__.py:37:80: E501 line too long (91 > 79 characters)
./datadotworld/hosts/__init__.py:38:80: E501 line too long (106 > 79 characters)
./datadotworld/hosts/__init__.py:39:80: E501 line too long (97 > 79 characters)

Thanks for taking the time to finish up this PR!

[laconc]

For this, what do you think about rewriting create_url to just require the subdomain? So this line would look like:

API_HOST = environ.get('DW_API_HOST', create_url('api', DW_ENVIRONMENT))

And then create_url could be something simpler like:

def create_url(subdomain, environment):
    if environment:
        subdomain = subdomain + '.' + environment

    return 'https://{}.data.world'.format(subdomain)

[kndungu]

My pleasure @laconc!

Issues fixed and suggestion applied.