`tsconfig.json` not being loaded by default

[arthurfiorette]

To reproduce this behavior:

git clone https://github.com/TypeStrong/typedoc
cd typedoc
npx ts-json-schema-generator -p src/**.ts -t ProjectReflection -f tsconfig.json --no-type-check >/dev/null  # succeeds
npx ts-json-schema-generator -p src/**.ts -t ProjectReflection --no-type-check >/dev/null  # fails

npx ts-json-schema-generator -p src/**.ts -t ProjectReflection -f tsconfig.json  # a few errors
npx ts-json-schema-generator -p src/**.ts -t ProjectReflection # a ton of errors

Originally posted by @pawamoy in #2197