Fix for incorrect identifier parsing in haskell-indentation-peek-token

The Haskell 98 standard specifies identifiers as 

    varid -> (small {small | large | digit | ' })

However, the haskell-indentation-peek-token didn't allow digits as follow-up characters in identifiers. This meant that for example

    in3 x = 3 `elem` x

did not parse correctly even though it is correct Haskell code.

Fix is simple: Use `:alnum:` instead of `:alpha:` in regular expression.