v1.0.3

fix nil pointer for parsing xlsx sharedStrings.xml file.
add some warn log for docx.

v1.0.2

enhance error tolerance of XLS file parsing.

v1.0.1

add image extraction APIs for pptx and xlsx

v1.0.0

Basically complete the file text content extraction functions.

Mainly support file formats:

• docx
• xlsx
• pptx
• pdf

Additional simple support:

• doc
• xls
• ppt