Add function to specify column widths: cols_widths()

[rich-iannone]

This PR allows for manual adjustments to column widths via the new cols_widths() function. We choose which columns get specific widths (in pixels, usually by use of the px() helper function) and all other columns are assigned a default width value though the .others argument.

There are two ways make the width assignments, both occur in the ...: (1) named arguments, where the names are those of the columns and the values are the dimensions (in px where, for instance, px(80) would helpfully generate 30px), and, (2) paired vars()---px() expressions, which:

• are separated by commas (e.g., vars(col_b), px(120))
• allow the use of one or more column names, column indices, or select helpers (e.g., starts_with(), contains(), everything(), etc.) within each vars() expression
• cannot be mixed with named arguments as in (1)

Here are two examples (using the two different expression styles) that make the same table:

# Use `exibble` to create a gt table;
# with named arguments in `...`, we
# can specify exact the widths for
# those columns (the `.others` value
# will provide a default width for
# any unset columns)
exibble %>%
  dplyr::select(
    num, char, date,
    datetime, row
  ) %>%
  gt() %>%
  cols_widths(
    num = px(150),
    char = px(100),
    date = px(200),
    datetime = px(200),
    .others = px(60)
  )

# We can take a different approach
# in making the same table by using
# paired `vars()`-dimension
# expressions; these are all
# unnamed and we need to ensure
# alternation between `vars()`
# and `px()` expressions
exibble %>%
  dplyr::select(
    num, char, date,
    datetime, row
  ) %>%
  gt() %>%
  cols_widths(
    vars(num), px(150),
    vars(ends_with("r")), px(100),
    vars(starts_with("date")), px(200),
    .others = px(60)
  )

Here is the table:

Fixes #111.