Allow specifying custom date format to yearmonth(<character>)

[mitchelloharawild]

This PR also moves the custom formats to the back of the list, which resolves #250.

library(tsibble)
#> 
#> Attaching package: 'tsibble'
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, union
yearmonth("Jan-20", format = "%b-%y")
#> <yearmonth[1]>
#> [1] "2020 Jan"

^{Created on 2021-03-25 by the reprex package (v1.0.0)}

[earowang]

Thx