allow colformat_md to offset citation note number

[atusy]

• Fix order of citations
  • Currently, automatic citations is done via YAML metadata block, but it has higher priority than body text.

---
output:
  html_document:
    keep_md: true
bibliography: example.bib
csl: https://raw.githubusercontent.com/citation-style-language/styles/master/vancouver.csl
---

@example1

```{r, echo=FALSE}
writeLines(
  sprintf(
    "@Book{example%s, author = {author%s}, title = {title}, year = {2021},}\n",
    1:3, 1:3
  ),
  "example.bib"
)
library(ftExtra)
data.frame(x = "@example2") %>%
  as_flextable() %>%
  colformat_md(.cite_offset = 1)
```

@example3

[atusy]

Another problem is the assumption that citations in the table do not appear earlier in the body, which is not always true.
I think this current strategy is not enough powerful despite of dirty hacks...
I close this PR and possibly reconsider in the future...