UI improvements and refactoring

[elpinal]

User Interface improvements

Number input

• remove leading and trailing whitespace
• on the non-Windows environments, there was a bug where the input's trailing \n is not removed

Both number & string input

• abort dialog if scanning from stdin failed
• properly put a newline if scanning failed
• Ctrl-D, which causes EOF, comes to end up a user input

Main program

• delay creating an output file and, if any, its parent directory so that when replacement generates any error, any files and directories are not created
• correct a number that the MIT license is assigned: as it was, it caused panic
• let users know the names of the licenses after they input a number; this change makes them assured because users may mistakenly input an accidental number

Others

• arrange some outputs mainly when they report errors
• fix typos
• refactor

Notes

• #6 is seemingly correct, but actually bufio.ReadLine ignores EOF if some bytes are already read (e.g., when a user types abc<C-d><C-d>, where <C-d> is Ctrl-D, the <C-d> behaves as if it is the enter key; abc is returned and error is nil)