Implemented Morse code and Restricted enciphering only to alphabets and digits

Added `encode_morse()` and `decode_morse()`.

Changed 
`[x.capitalize() for x in pt if not(x.isspace() or x=="." or x=="!" or x==",")]`
to
`[x.capitalize() for x in pt if if (x.isalpha() or x.isdigit())]` 
so that only alphanumeric characters are encoded.