Add import/export functionality

[psvenk]

Description: This PR adds two command-line options, json and out. The json option allows the user to specify a JSON file to read from (instead of scraping Aspen), similar to running ./serve.js fake but with control over what file is used. The out option allows the user to specify a file to which JSON containing information about the user's grades, schedule, etc. is dumped. This JSON file can be read later by using the json option with Aspine's serve.js. Additionally, the arguments insecure and fake are now accepted both as arguments and as options (so ./serve.js --insecure is equivalent to ./serve.js insecure).

Examples:

# Load sample file, equivalent to "fake" argument
./serve.js --json=./public/sample2.json

# Dump data into 2019-10-03.json
./serve.js --out=2019-10-03.json

# Read from 2019-10-03.json instead of scraping Aspen
./serve.js --json=2019-10-03.json

Use case: Aspen only allows students to see their current year and the previous year, thereby stopping students from viewing their grades from two years ago. This feature will make it much easier for students to archive their gradebook. Additionally, graduating students will be able to save their gradebook for future reference in case their accounts are terminated upon graduation.

Future additions: Some features that might be nice to implement down the road:

• Import/export functionality from the Web interface, so that the user won't have to host Aspine on their own computer to backup their Aspen data
• Add the Aspine version number to the JSON so that users will know what version of Aspine to download if a future version breaks compatibility
• Add support for "previous year" grades in Aspine (relevant here because the previous academic year would be included in exports)