Adding line numbers to the output of mirb.

Before:

    > "hi"
    hi
    > d
    (mirb):1: undefined method 'd' for main (NoMethodError)
    > d
    (mirb):1: undefined method 'd' for main (NoMethodError)
    > "hi"
    hi
    > "#{'}"
    line 1: unterminated string meets end of file

After

    > "hi"
    hi
    > d
    (mirb):2: undefined method 'd' for main (NoMethodError)
    > d
    (mirb):3: undefined method 'd' for main (NoMethodError)
    > "hi"
    hi
    > "#{'}"
    line 5: unterminated string meets end of file