Signed-off-by: NMartin Schwenke <martin@meltin.net>

Specifies the maximum severity of errors to handle.  For example,
specifying "-S warning" means that errors of severity "info" and
"style" are ignored.
Signed-off-by: NMartin Schwenke <martin@meltin.net>

This does the necessary work to emit end columns on AST analyses. SC2086
is made to emit a correct end column as an illustrative example.

For example:
```
$ shellcheck -s bash -f json /dev/stdin <<< 'echo $1'
[{"file":"/dev/stdin","line":1,"endLine":1,"column":6,"endColumn":8,"level":"info","code":2086,"message":"Double quote to prevent globbing and word splitting."}]
```

This change deprecates the parser's getNextId and getNextIdAt, replacing
it with a new withNextId function. This function has the type signature:

withNextId :: Monad m => ParsecT s UserState (SCBase m) (Id -> b) -> ParsecT s UserState (SCBase m) b

Specifically, it should be used to wrap read* functions and will pass in
a newly generated Id which should be used to represent that node.
Sub-parsers will need their own call to withNextId in order to get a
unique Id.

In doing this, withNextId can now track both the entry and exit position
of every read* parser which uses it, enabling the tracking of end
columns throughout the application.

Also update the ShellCheck homepage to the new dedicated domain.

This change makes PositionedComment and ParseNote contain end columns.
It additionally modifies the JSON formatter to show the end column in an
"endColumn" property. No modifications to the messages shown by any
other formatter have been made.

Currently, all checks set the end column to the start column. It should
now be possible however to start setting the end column in the parser.
Additional work is needed to set the end column during AST analysis.