Given a string containing letters and + characters implement a function which determines if each letter in the string
is surrounded by + character. There may be more than one + character between letters (+a++b+) and letters may be
surrounded by the same + character (+a+b+).
| Challenge | Solution |
surroundedLetter("+a+") // true
surroundedLetter("+ab+") // false
surroundedLetter("+a+b+") // true
surroundedLetter("+a++b++") // true