read_meme fails when alphabet is DNA/RNA/AA-LIKE or custom

[Spencer Nystrom]

dreme.txt

> read_meme("dreme.txt")
Error in strsplit(alph, "\\s+")[[1]] : subscript out of bounds

This happens when the ALPHABET string defines a DNA/RNA/AA-LIKE or custom alphabet.

A few outcomes could happen here:

1. Throw informative error for both *-LIKE alphabets and custom alphabets
2. use *-LIKE Alphabets as DNA/RNA/AA
3. throw an error for custom alphabets
4. fully support custom alphabets

4 is quite the undertaking, so I've implemented 2 & 3. PR incoming.