Add more powerful bs4col() and bs4row() functions

[epruesse]

Bootstrap 4 allows unsized columns (equal width), naturally sized columns (auto), vertical alignment of column content and horizontal justification of columns. This concept PR would add two functions to set the appropriate classes.

As is, the column() and fluidRow() functions are actually somewhat less convenient than using div(class="col-sm-4", ...) directly.

Semantics are different w.r.t. column - the width is not necessary, might have only content and the columns just be set to equal width default.

I can write docs for this if there is interest. If not, feel free to close PR.

[epruesse]

bs4row(
    justify = "center",
    bs4col(
         width.lg = "auto",
         "as wide as the text"
    ),
    bs4col(
         width.xs = 2, width.lg = 6,
         class = "some_extra_class",
         align = "center",
         "half the space",
    )
),
bs4row(
    bs4col("equal"), bs4col("sized"), bs4col("columns")
)