Multiline strings should preseve endlines

[marshallward]

This is an example multiline namelist (from the testing).

&string_multiline_nml
empty = '

'
trailing_whitespace = ' 
 
'
/

Although f90nml can handle these, it drops the endline characters.

I don't know if this was intentional at the time (the commit log seems a bit ambiguous: 46f9bd3), nor am I sure what the defined behavior would be, but it does not seem to agree with GFortran, which preserves the endlines.

Unless I see otherwise, the tokenizer should be modified to preserve endlines in multiline strings.

(This was detected and fixed in a development branch with a new token scanner)