First draft to support the following CSS properties:
grid-row-start, grid-row-end, grid-column-start, grid-column-end,
grid-row, grid-column and grid-area.

Add a `4` at the end of such data types and functions for more
consistency.

This avoids a warning from the compiler.

Use of folding headings to make the documentation more compact.

Added a Example heading. There is unfortunately a bug with Haddock
when trying to make the heading foldable.

This will allow a more finer control grain over the types and
there documentation.

The checks are for the moment only when the custom-ident is
created as a 'CustomIdent' type.

As long as there are no function implementing Either instead
of the partial functions, there is a risk for invalid value being
built by the user. Also we could later change the interface of
those functions/constructors so they return an Either value.

Errors are now raised when invalid custom-ident are provided,
regardless if they are provided as a String or as a CustomIdentGrid.

The compilation error is with older version of GHC when compiling
with Nix.

Comments of arguments where not properly formatted and lead to a
compilation failure for older version of GHC when testing the build
with Nix.