Improved `s:EscapeCtagsCmd()` for windows `cmd.exe` to eliminate issues when `a:ctags_bin` calls a batch file that uses `%~dp0`. Changes include:

* Enable ctags_args to be a list
* Changed default ctags_args to be a list to improve robustness of
  s:EscapeCtagsCmd()
* Do not escape 0th argument of ctags_cmd for cmd.exe when it is not
  necessary because otherwise batch files may not return expected value
  for %~dp0
* Updated tagbar documentation about string and list value types for
  ctags_args

See https://github.com/majutsushi/tagbar/issues/133