Add spanner id as an argument to tab_spanner()

[rich-iannone]

This PR allows for the setting of a spanner ID value, which makes it easier to access a spanner column label through cells_column_spanners() (when using tab_style() or tab_footnote()]), it occurs through the idvalue (and not thelabel). If an id` is not provided here, it will be generated from the label, stripping any tags and special characters, and limiting the generated string to 10 characters.

Although a message will be provided in this case, it is advised that an explicit id value be set if planning to access this cell in a later function call. Also, when providing an id, it must be unique across all ID values set for column spanner labels.

Fixes: #521
Fixes: #626
Fixes: #597

[jcheng5]

After discussing in person:

1. No restriction on id; just use the label as-is by default.
2. Don't attempt to automatically fix duplicate id's in tab_spanner; just stop with an error message informative enough that it's clear how they should proceed.
3. For tab_spanner_delim, automatically fix duplicate id's using TBD method (tibble::name_repair?) and if that was necessary then let user know what the resulting id's are. Maybe we should have a param that lets them turn the message off.
4. If we allow n-level spanner hierarchies I'd like to revisit this, I'd imagine uniqueness only mattering within a single scope (like filesystem hierarchies).

[jcheng5]

Just the nits from the comments

[jcheng5]

1. RTF needs to use spanner id's instead of labels.
2. rle() needs to use spanner_id instead of spanner.
3. Test case with adjacent spanners that use same label but different IDs; they should not be merged into one spanner.

[rich-iannone]

• RTF needs to use spanner id's instead of labels.
• rle() needs to use spanner_id instead of spanner.
• Test case with adjacent spanners that use same label but different IDs; they should not be merged into one spanner.

All of these items are addressed now and I took the time to fine tune the docs for tab_spanner() and tab_spanner_delim() as well.

[jcheng5]

1. Test cases with spanner id != spanner label, for HTML, latex and rtf
2. More test cases for latex and rtf