BBcode venue

[mcanouil]

As discussed in #350, here is the PR to add BBcode support (mostly for myself, I guess)

(reprex(c("#' Hello world", "## comment", "1:5"), venue = "bb"))
#> i Rendering reprex...
#> v Reprex output is on the clipboard.
#> [1] "[code=php]"                                          
#> [2] "c(\"#' Hello world\", \"## comment\", \"1:5\")"      
#> [3] "#> [1] \"#' Hello world\" \"## comment\"     \"1:5\""
#> [4] "[/code]"                                             

Currently, the BBcode output will look like this:

Hello world
[code=php]
## comment
1:5
#> [1] 1 2 3 4 5
[/code]

This PR only translates markdown R code chunk and markdown image tags to BBcode.

Edit: I'll add other common tags such as bold/italics.

[mcanouil]

I don't think there is need to add other BBcode tags for a reprex (list, url for example).
If those translations are not needed, I think this PR is "complete".

(reprex(
  input = c(
    "#' Hello world",
    "## comment",
    "1:5",
    "#' Next is _italic_ or **bold** or even __*bold italic*__",
    "plot(1:3)"
  ), 
  venue = "bb"
))
#> i Rendering reprex...
#> v Reprex output is on the clipboard.
#>  [1] "Hello world"                                                           
#>  [2] ""                                                                      
#>  [3] "[code=php]"                                                            
#>  [4] "## comment"                                                            
#>  [5] "1:5"                                                                   
#>  [6] "#> [1] 1 2 3 4 5"                                                      
#>  [7] "[/code]"                                                               
#>  [8] ""                                                                      
#>  [9] "Next is [i]italic[/i] or [b]bold[/b] or even [i][b]bold italic[/b][/i]"
#> [10] ""                                                                      
#> [11] "[code=php]"                                                            
#> [12] "plot(1:3)"                                                             
#> [13] "[/code]"                                                               
#> [14] ""                                                                      
#> [15] "[img]https://i.imgur.com/6dmuUEo.png[/img]"