BUG/ENH: Fix to_excel representation of inf values
Fixes #6782 by converting inf values to string (default is `'inf'`).
This still round trips successfully. According to Stack Overflow post I
found, there isn't a specific representation of `INF` values -
http://stackoverflow.com/questions/15704538/how-can-i-express-nan-values-in-excel
so we're going to make the decision _for_ the writers. This also
(implicitly) fixes an issues where round-tripping an Excel file with an
inf value would cause an `OverflowError` and fail to read back.

cc @jmcnamara and @arthurgerigk-rocket

should also handle -inf
 @jreback good call - any preference between `not np.isfinite(val)` and `abs(some_value) == np.inf`?
 I think u should use isinfinite maybe has better platform compat (or may be the same)
the preprnd a minus to inf_rep  if negative?
 Yeah, realized that after I said it haha
 okay, that's done.
 ok looks fine

side note - this formatting to excel space should be done by blocks - would make it much faster (but separate issue)
 that's a good point. - let's accept this for now (along with the extra tests to spec out inf) and then we can go back to that later on.
