Instruction to run the program: (Python 3.6 required)
    1. Go to directory where merge.py file is placed.
    2. Run chmod a+x merge.py
    3. Run ./merge.py -i {full path to input directory} -o {full path to output file}
    4. Given that each input files are all sorted(excluding blank lines), will generate merged output file.
    5. If any of the file contains non lexicographically sorted line, Exception will be thrown.


Implementation Detail:
    - Simple Argparse has been used to gather 2 parameters.
    - Iterator class(MergeIterator) to simulate Merging Sorted Inputs routine.
    Iterator takes care of each input file handle opening/closing. Python Context Manager could have been
    used alternatively to handle clean up and it's a design choice.


Complexity Analysis:
Run: Assuming there are N total input files with average size of K lines..
    Min Heap's size at one time won't go over N elements, and the program will execute
    N*K amounts of Heap Push & Pop each taking logN time.
    Thus, the worst case runtime complexity is O(NKlgN).
    Total runtime complexity:
        O(NKlgN - heap operation) +
        O(N - file gathering & opening/closing) +
        O(NK - line writing)
Space: We're reading each files' line one by one and at one time would have read 1 inputs from N files each.
    So the space complexity is: O(N - heap size)