Provide a way to do multi-line input in the command line client

Provide a way to do multi-line input in the command line client. Typically this is done by introducing a client-only 'separator' command, which allows to change the current line separator.

By default, the separator is newline. If someone changes it to, for example, \go, the client gobbles up the input, ignoring newlines, until it meets \go, and only then sends it to the server.

The separator itself is excluded from the query and not sent to the server.

This is useful to be able to input longer-than-line Lua commands, loops, in the command line client.

An alternative way to implement the feature is to add Lua interpreter to the command line, and check syntax of the input: only when the input makes up a valid lua chunk, it is sent to the client. This alternative has its own issues.