Option to force uppercase SQL keywords

[othyn]

This adds a new optional third parameter to the format method, $forceUppercase which defaults to false to maintain backwards compatibility.

When passed as true, all keywords will be uppercased.

There is a new test written in for this functionality that passes.

Fixes: https://github.com/doctrine/sql-formatter/issues/32. Implementation abstracted from the original PR against the forked source: https://github.com/jdorn/sql-formatter/pull/86