Parent initializes by existing iframe

[aiji42]

Issue

#4

What I did.

• updated src/Parent.ts
  • Changed the argument type to Union.
    • type IConstructorArgs = IConstructorByUrlArgs | IConstructorByElementArgs;
  • Changed to initialize by conditional branching from argument types.
    • This is verbose, so it may be better to split it into several functions.
• created examples/parentExistingIframe.html

What I didn't do.

• No change src/index.test.ts / src/integration.test.ts.
• The README has not been changed.
  • I will respond if changes are needed.