Constrain width of column headers based on terminal size

Imperfect algo to start
First pass at an algorithm to constrain row values based on width
Update algo to split remaining extra width amongst the large ones
Make padding a class-level variable
Constraint algo needs to accommodate character widths
Introduce `getDisplayLines()` which will be useful for testing
Ability to set constraint width on renderer
Merge branch 'master' into constrain-table-43
`row()` can return `PHP_EOL` delimited strings

Let's cleanly handle it
Test coverage for table constraining
Check the table height too
Fix typo
Rename variable
`null` doesn't actually work until PHP 5.4.12

See https://github.com/php/php-src/pull/133
Merge pull request #43 from wp-cli/constrain-table-43

Ability to have multiple lines in a cell in Table