cli: support --bannerComment as file input

[Hotell]

Currently there is no way how to pass to CLI file with banner ( banners tends to be longer than one line comment )

Current solution:

json2ts -i events.json -o src/Events.ts --bannerComment "$(cat ./banner.txt)"

Proposed feature:

# Will use inline string for banner
json2ts -i events.json -o src/Events.ts --bannerComment '//Hello darnkess my old friend'

# Will use content of file
json2ts -i events.json -o src/Events.ts --bannerComment ./banner.txt

If you're ok with that I can send PR.

[bcherny]

Sounds great to me!