Add export and import to file (efficiently)

[jonsnowseven]

This is more a feature request than an issue (sorry if it is not the correct place to report this).

Description

I think it could be very useful to have a functionality that exports and imports efficiently a bounter object to and from a file.

Something like:

from bounter import CountMinSketch

cms = CountMinSketch(size_mb=xxx)
cms.update(['a', 'b', 'c', 'a'])
cms.export('file')
cms = CountMinSketch(file='file')
print(cms.counts['a'])

I believe this can be accomplished with this:

import pickle
from bounter import CountMinSketch

cms = CountMinSketch(size_mb=xxx)
cms.update(['a', 'b', 'c', 'a'])
with open('counter.pkl', 'wb') as f:
    pickle.dump(cms, f)

with open('counter.pkl', 'rb') as f:
    cms = pickle.load(f)

print(b.size())
print(b['a'])

But maybe there is a more efficient way to export the object.

Thank you in advance.