Add custom variable to change error highlighting [#35]

Remove `flycheck-ignore-columns` in favor of
`flycheck-highlighting-mode` which provides a more flexible and future
proof approach.

`(setq flycheck-ignore-columns t)` hence becomes `(setq
flycheck-highlighting-mode 'lines)`.
Document error navigation
De-qualify checker symbols

Remove the `flycheck-checker-` prefix from all syntax checkers.  We do
not risk any name collision by doings so, because checker symbols are
not used as functions or variables, but only to attach symbol properties
which are properly qualified.
Attach configuration file variable to checker symbols
Add a getter for the major modes of a checker
Add a command to show help for a checker
Add documentation to checkers
Fix casing
Merge pull request #52 from lunaryorn/checker-documentation

Add documentation to checkers
Fix dead references
Fix setup examples

Enable "Flycheck" instead of "flymake" and do not recommend use of obsolete functions.
Fix display of predicate in checker help
Add getters for checker properties

Also use `documentation-symbol` to extract documentation from checkers
in a standard way.
Refine checker help output
Complete with *all* checkers when reading from minibuf

Also require input to match the completion list to avoid input of
non-checker symbols.
Restore next-error-function when disabling flycheck-mode
Signal an error if we can't navigate further
Use `user-error` appropriately
Declare customizations first
Fix taglines of custom groups