Add support for multiline messages

[d-uzlov]

Currently when a message has many lines, most of the lines get printed as is, without date, level, fields, caller, etc.
Example:

ll.Info(`line 1
line 2
line 3`)

Output:

- [INFO] [test:field] line 1
line 2
line 3

With new option MetaPerLine set to true the output is more consistent:

- [INFO] [test:field] line 1
- [INFO] [test:field] line 2
- [INFO] [test:field] line 3