NPOI.Extension Versions Save

1. Support fluent configuration api;
2. Support more custom cell formartters;
3. Fixed some issues;

Features:

1. Support fluent api, decouple the configuration from the POCO model by using fluent api. (doesn't poison your POCO)

Fixed bugs:

1. Fixed the cells merge behaviors crash when the will be merged cell's value isn't string value.

hotfix the file exist check logic error

1. Support export to exist exel file;
2. Support xls, xlsx;
3. Support multiple sheets;

IEnumerable<T> save to and load from excel supports T is POCO

The extensions of NPOI, which provides IEnumerable<T> save to and load from excel functionality.

Following functionality in this pre-release:

1. Use attribute to control excel column name, and cell index;
2. Use attribute to control excel cells SUM and cells MERGE behaviors;
3. Use attribute to control excel filter behaviors
4. Use attribute to control excel freeze behaviors