Refine code

[niszet]

exported functions have 3 types of input:

• docx file path
• xml object
• data.frame

These are easy to get confused. functions must have prefix and any other mark in those names.

[niszet]

Added "file" to the docx related argument. i.e. read_styles, write_styles.
In this package, the name xml should not be used. Use node instead. Or, docx_styles, styles_node, style_nodes and so on.

For data.frame, use df as a name of argument.

Suffix or prefix of function names are under consideration...