Draft new version using R_user_dir()

[maelle]

Notes

• I use @examplesIf to ensure the installation example will never be run on CRAN.

TODOS

• [ ] Look for examples of CRAN packages using this;
• [ ] Test on various platforms (R-hub builder is currently down).

[maelle]

To escape examples, I wonder whether there's an exported function checking whether tika is available, it seems not?