Adding configuration option

[mkim219]

What I have done

• Add option program.option('-c,--config','JSON configuration file');
• Add try and catch statement for checking existence of JSON file. if JSON file does not exist it throws error
• I have set up default value of input and output in the try and catch statement
• Create a variable "isJson" and function isJSON for checking whether input is JSON file or not
• Create function called generateHTMLFromFile(input, output) and generateHTMLFromDir(input, output) based on your code
• If the input is single text or md file from JSON configuration file, the generateHTMLFromFile function will trigger otherwise generateHTMLFromDir will trigger
• I have set up the default parameter in case using input without JSON configuration file

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[mkim219]

Please test the change that I have made and let me know if there is an issue.