Add an option to control the cell trimming.

[yasezbk]

This is useful when cells contain text where indentation is important (like JSON or XML).

This change preserves the existing behaviour (both left and right trimming is done by default).

Example output from no_trim sample:

+----------------+
| This is        |
| a cell that    |
| spans          |
| multiple lines |
+----------------+
| {              |
|   foo          |
|   bar          |
| }              |
+----------------+