Introduce options

[gagolews]

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 (Warn (default=FALSE, change via option()) if input is an empty vector #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 Allow stri_locate_first's output to be consistent with that of stri_locate_all #424