add optional `limit` kwarg to parameterize fetch size & handle pagination

[otosky]

adding a hotfix for this minor version to enable query fetch size to be supplied as an argument, as well as finally handling Medium API response pagination

New functionality

For a User:

from medium_stats.scraper import StatGrabberUser

me = StatGrabberUser('username', sid='sid', uid='uid', start=start, stop=stop)

# fetches 50 stories per request as default, but paginates through all stories
me.get_summary_stats() 

# will fetch 1 story per request, but paginates through all stories
me.get_summary_stats(limit=1)

For a Publication:

from medium_stats.scraper import StatGrabberPublication

pub = StatGrabberPublication('slug', 'sid', 'uid', start, stop)

# fetches 50 stories per request as default, but paginates through all stories
pub.get_all_story_overview()  

# will fetch 1 story per request, but paginates through all stories
pub.get_all_story_overview(limit=1)  

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contributes to #10