Reorganise the API

[nacnudus]

With more functionality, it makes sense to split things up into separate functions.

• xlsx_cells() cells of all worksheets in one data frame (option to get "data" only #16)
• xlsx_formats() formatting and styles (option to get "data" only #16)
• xlsx_validation() data input validation rules, per sheet (Data Validation #6)
• xlsx_names() named formulas/ranges (Add Named regions to returned data #17)
• xlsx_sheet_names() utility function

The xlex family:

• xlex() tokenize a formula
• xlex_edges() and xlex_vertices() operate on the parse tree
• plot_xlex() visualise the parse tree
• demo_xlex() shiny app of the above

Utility functions for common tasks:

• Formatting that takes advantage of tab completion for discovery, using very long function names like fill_patternFill_fgColor_rgb(x, scope = c("style", "local")) or border_horizontal_style_indexed(x, scope =c("style", "local")), where xis a data frame withstyle_formatandlocal_format_id` in it.

• A way to extract specific styles from all formats, e.g. bad <- get_style(x, "bad") where x is the return value of xlsx_formats().