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
Removed unnecessary | around 'shell'
Merge pull request #135 from darcyparker/issue133_ctagsbin

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