Development of API-based errant_compare

[gotutiyan]

Hi, as can be seen in the issue #25, I think API-based errant_compare feature will be useful for many people.
I also want the feature so I created a pilot version.
Mainly,

• Overall, the implementation does not use coder_dict, but uses the Edit object directly.
• Created errant/compare.py and implement the API-based errant_compare
• Added __eq__() function to the Edit class to compare two objects easily.
• All of options used in the official are supported (i.e. -cs, -ds, -dt, -cse, -single, -multi, -cat and -filt)

Using the GEC system's correction results at hand, I confirmed that the results of the official implementation match the results of my implementation.

I do not think this PR will be merged as is, but I hope this is a clue to development of API-based errant_compare!