Clarify usage of parameters when using CSV from file #65

It seems pantable understands parameters like caption, alignment, etc. in combination with CSV from a file only if they are in the markup or in the file, not in the element meta-data.

As putting them into the file will make the file unusable for most other tools, this is a bit unfortunate.

Approach 3, i.e. putting the meta-data including the include parameter into the code block is actually fine, but it should be documented more clearly if possible. Thanks!

This works: Approach 1

```{.table include="data/csv_example_with_header.csv"}
```

File: csv_example_with_header.csv

---
caption: '*Awesome* **Markdown** Table from `pantable`'
alignment: RC
table-width: 2/3
markdown: True
---
prefix, city_or_region, comments, Status
'030', 'Berlin', 'My comment', True
'069', 'Frankfurt', , False
'089', 'Munich', 'Another comment', True

This does not work (caption etc. missing): Approach 2

```{.table include="data/csv_example.csv" alignment="RC" caption="A cool CSV Table"}
```

This also works: Approach 3

```table
---
caption: '*Awesome* **Markdown** Table from `pantable`'
alignment: RC
table-width: 2/3
markdown: True
include: "data/csv_example.csv"
---
```

File: csv_example.csv

prefix, city_or_region, comments, Status
'030', 'Berlin', 'My comment', True
'069', 'Frankfurt', , False
'089', 'Munich', 'Another comment', True

Environment:

pandoc                    2.17.0.1             
pandoc-crossref           0.3.12.2           
pantable                  0.14.2