Support multi-character sep argument

[richierocks]

I want to write:

fread(
  "x<SEP>y\n1<SEP>a\n2<SEP>b",
  sep = "<SEP>"
)

This gives the error message

Error in fread("x<SEP>y\n1<SEP>a\n2<SEP>b", sep = "<SEP>") : 
  'sep' must be 'auto' or a single character

Real-world example of a dataset that uses <SEP> as a column separator:

http://labrosa.ee.columbia.edu/millionsong

[MichaelChirico]

8 years later and only one other community request for this... I suspect it will be nontrivial to support this & with potential performance impacts for the 1-character case.

PRs welcome, but for now, pumping such files through sed or some other command-line tool first to convert the multi-character sep= into a one-character alternative seems good enough.