Support custom single line comment notation

[ymotongpoo]

Runtime environment

• OS (version): any
• Hardware: any
• VS Code version: any
• VS Code build number: any
• other extensions installed: many

Behaviors

Expected behavior

In plain text, usually we don't have consistent comment style, but we, engineers, often borrow comment styles from programming languages.
It would be nice if the users can set their own preferred comment notation in the configuration.

Actual behavior

Fixed comment style hard coded in src/notations.ts is used.

Steps to reproduce the issue

1. Write text in the file with extension recognized as "plaintext" by VS Code.
2. Run command of licenser "insert license header."