Get user corresponding to the specified username
Examples
# \donttest{
# Retrieve information about the jl user
get_user_by_username("jl")
#> List of 5
#> $ about : chr "This is a test"
#> $ created : POSIXct[1:1], format: "2007-03-15 01:50:46"
#> $ id : chr "jl"
#> $ karma : int 4307
#> $ submitted: int [1:850] 35686379 35675818 25172559 25172553 19464269 18498213 16659709 16659632 16659556 14237416 ...
#> - attr(*, "class")= chr "hn_user"
# }