Function idea: use_potools() for initial setup

[maelle]

When starting to use potools in a package, the first step is to register the potools usage style in DESCRIPTION.

Could there be a function for doing this, use_potools(style = "explicit") (so the explicit style would be the default one)?

And if the style is explicit, then the function would also create R/utils-potools.R with the definition of tr_().

Then the function would indicate the next steps: wrap text in tr_(), run po_extract() and po_create().

[MichaelChirico]

Related: #39