v0.2.0

feature #9 Allow optional value as NULL when default is set (sstok)

This PR was merged into the master branch.

Discussion
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|Q            |A  |
|---          |---|
|Bug Fix?     |n  |
|New Feature? |yes|
|BC Breaks?   |yes|
|Deprecations?|no |
|Tests Pass?  |yes|
|Fixed Tickets|   |
|License      | MIT |

When interactive-mode is enabled for all questions it's not possible to leave an optional value empty.
The SymfonyStyle forces that a value is required and even then when there is an default leaving the value empty, will use the default!

This change makes it possible to mark a value as option by treading some values as NULL.
For example:

* `.` for the `src_dir` will transformed to NULL.
* `*` for php_cs_finder_name will be transformed to NULL.

Note that a question marked as optional requires eg. a default or value for the `markOptional` method.
Else an exception gets thrown.

Commits
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1116e9013dfca264b4df3aaeae92ce4d4e688df4 Allow optional values as NULL when default is set