Reader writer flags

[MarkBaker]

This is:

- [ ] a bugfix
- [X] a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

Improved use of Readers/Writers by allowing passing flags in the load()/save() methods to support functionality like includeCharts, preparatory to adding support for pivot tables with the same optional mechanism.

Examples:

$reader = IOFactory::createReaderForFile("spreadsheetWithCharts.xlsx");
$reader->load("spreadsheetWithCharts.xlsx", $reader::LOAD_WITH_CHARTS);

or

$spreadsheet = IOFactory::load("spreadsheetWithCharts.xlsx", IReader::LOAD_WITH_CHARTS);

The same functionality can also be applied to other "boolean" features like readDataOnly. Once those features are enabled in this manner, it will be possible to combine multiple flags in a load request:

$reader = IOFactory::createReaderForFile("spreadsheetWithCharts.xlsx");
$reader->load("spreadsheetWithCharts.xlsx", $reader::LOAD_WITH_CHARTS | $reader::LOAD_DATA_ONLY | $reader::LOAD_WITH_PIVOT_TABLES);

or

$spreadsheet = IOFactory::load("spreadsheetWithCharts.xlsx", IReader::LOAD_WITH_CHARTS | IRreader::LOAD_DATA_ONLY | IReader::LOAD_WITH_PIVOT_TABLES);