Support omitting first and last corner

Many MarkDown flavours - and lazy people alike - prefer not to have the `|` characters added at the sides of tables.

In other words, instead of this:

```
| foo | bar | baz |
| 1 | 2 | 3 |
```

one would use:

```
foo | bar | baz
1 | 2 | 3
```

However, at the moment if using the latter format table-mode would not recognise the table - at least not for the `\tr` mapping and `:TableModeRealing` in general.

Can this feature be added perhaps?