Bash-Support: Default shell can be set, QuickFix list works properly

- Default shell can be set in ~/.vimrc via g:BASH_Executable.
- Errors can be parsed for bash, dash, ksh, zsh.
- Bugfix: QuickFix list doesn't work properly (thanks to @PengZheng)
- 'Run -> make script executable' (\re) is now a toggle.
- Minor corrections and improvements.