Support sam's structural regular expression based command language

For those not familiar with sam(1) more information can be found at

 http://sam.cat-v.org/

For now sam commands can be entered from the vis prompt via :sam <cmd>

A command behaves differently depending on the mode in which it is issued:

 - in visual mode it behaves as if an implicit extract x command
   matching the current selection(s) would be preceding it. That is
   the command is executed once for each selection.

 - in normal mode:

    * if an address for the command was provided it is evaluated starting
      from the current cursor position(s) i.e. dot is set to the current
      cursor position.

    * if no address was supplied to the command then:

       + if multiple cursors exist, the command is executed once for every
         cursor with dot set to the current line of the cursor

       + otherwise if there is only 1 cursor then the command is executed
         with dot set to the whole file

The command syntax was slightly tweaked to accpet more terse commands.

  - When specifiying text or regular expressions the trailing delimiter can
    be elided if the meaning is unambigious.

  - If only an address is provided the print command will be executed.

  - The print command creates a selection matching its range.

  - In text entry \t inserts a literal tab character (sam only recognizes \n).

Hence the sam command ,x/pattern/ can be abbreviated to x/pattern

If a command is successful vis switches to normal mode (and hence removes
any selections), otherwise the editor is kept in visual mode. The print
command "fails" by definition.

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