Make R code auto-completion work within in-line R chunk

[jooyoungseo]

While code completion does work in block chunk, it does not work inside in-line R chunk.

Examples are as follows:

``` r
# Hit ctrl+spacebar to invoke auto-suggestion after typing the following line:
ggp
```

# The following does not work:
`r ggp`

[renkun-ken]

I think it is possible to support this. Just check if the document is Rmd and then if the cursor is not in a code chunk but inside a backtick quote that starts with `r then completion should be provided.