README: clarifications about style checkers.
Minor cleanup for text filetype.

- No default checker for text filetype.
- Checker `atdtool` needs sorting.
- Filetype `mail` is now an alias to `text`.
PC-Lint: add column numbers.  Cleanup.
PC-Lint: mark informational messages as "Style".
Manual: add a note about opening the error window.
Clear loclist when running lolder at the bottom of the loclist stack.

This saves people some confusion when syntastic_always_populate_loc_list
is unset and they try to open the error window with :lopen or :lwindow.
FAQ: add a note about empty error windows.
New options.

Add options b:syntastic_<filetype>_<checker>_exec and
b:syntastic_<checker>_exec.
Expand b:syntastic_<checker>_exec.
Puppet: fix errorformat.
Puppet: the "future" parser outputs column numbers.
More grouping in errorformat.
Add support for maven customizations