improve NA handling #closes #300

[JanMarvin]

Write na.strings with write_xlsx() and read na.strings() and na.numbers().

> df <- data.frame(
+   num = c(1, -99, 3, NA_real_),
+   char = c("hello", "99", "3", NA_character_)
+ )
> 
> test <- temp_xlsx()
> write_xlsx(df, file = test, na.strings = "N/A")
> 
> read_xlsx(test, na.strings = "N/A", na.numbers = -99)
  num  char
2   1 hello
3  NA    99
4   3     3
5  NA  <NA>