Additional formatting for p values

[GegznaV]

Can extra options be added to functions pvalue() and pvalue_format() to format p values more flexibly:

1. For big p values, can > be added, e.g., p > 0.999 instead of p = 1.000?
   Desired ouptut:

pvalue(0.999945)
## p>0.999

2. An option to add extra spaces around >, = or < would be useful too.
   E.g.:

pvalue(0.523)
## p=0.523

pvalue(0.523, spaces = TRUE)
## p = 0.523

An example where this feature could be used: kassambara/ggpubr#186

3. Possibility to remove leading zero (as this is a common practice in APA style statistical reports):
   E.g.:

pvalue(0.523, leading_zero = FALSE)
## p=.523

4. Add option to force = in the output even if add_p = FALSE

Can any of these ideas be implemented in scales package?

[hadley]

I did all of these (mostly via the new prefix argument) except leading_zero because implementing that look like it was going to be fiddly (since there's no equivalent in format()).