* fix(tags): fix rg command if PCRE not available (closes#145)
Some linux distro do not compile ripgrep using the --pcre2 flag.
This commit checks if the --pcre2 flag is available and then tweaks the
ripgrep command accordingly.
At the moment, it will therefore not be possible to filter-out
hexademical color codes from #tags for people that do not have the
--pcre2 flag available. This will be documented in a later commit.
**Important**: the detection of the PCRE availability is done by trying
a simple rg search using _os.execute_. This function interface was
changed in Lua 5.2, which means it will break when Neovim will update to
Lua >5.1. The fix is already there in commented form.
Fixes#145
* refact: cleanup
* doc(tag): add note about rg --pcre2
* doc(tag): add note about rg --pcre2 in vim help
