Introduce options

Some behaviour could be controlled by means of global options (in the sense of `options()`), because not everything is for everybody, ideal world does not exist, and de gustibus non est disputandum.

For example (any suggestions are welcome):

* `omit_no_match` now defaults to `FALSE` in `stri_locate_all_fixed`, but what if someone consistently prefers `TRUE`?
* operations on empty vectors result in an empty vector, but there could be a warning issued when this happens (#391)
* `omit_empty` in `stri_split`
* `ignore_null` in `stri_paste` is now `FALSE` but can be TRUE or trigger a warning
* `simplify` in `stri_split` and `extract` but see also #424

Review the existing options() behavior and the listed entry points: stri_locate_all_fixed, stri_split, stri_paste, and extract. Resolve the open choices, including the interactions with #391 and #424, then define the global-option behavior and its expected outcomes before implementation.

Written by the indexing model from the issue text.