add initial attempt at SQL chunk option immediate

Improved handling of options$immediate and additional options$replace - these edits work in a knitr::knit_engines$set( ) style test. 

Immediate = T passes param immediate = T to dbExecute() / dbGetQuery(). Immedate is not implemented for query's with options$max.print set. 
I can't be sure if options$immediate also works icw options$params - I can't get params to run at all on my server. Passing values from local environment using ?value works however, also icw the immediate option. 

Replace = T adds code to drop an existing temporary table before running the into # statement. I thought this might be handy but leave it to others to decide on whether this is truly needed (I also don't really know if this implementation works on other servers than the one I use).
I've also included an error message to catch attempted replacement of non-temporary tables but error handling doesn't work yet as expected - when attempting to replace a table with a name not starting with #, the generic server error message appears: ([SQL Server]CREATE TABLE permission denied in database 'DWH'. ) - perhaps someone else will now how to fix, though perhaps we'll not want to implement the replace option at all. 

Example code: 
```{r qvalue}
StadiumCode <- '0A'
```


```{sql, immediate = T, replace = T}
SELECT  * 
into #tum
FROM statistiek.vw_dimStadium
where StadiumCode = ?StadiumCode

```

```{sql fetch}
SELECT   *
FROM #tum
```

Fixed error handling on temporary table check. What a difference a space makes...

- added prefixes sql. to immediate and replace options
- set NULL as default for sql.immediate
- handling of user input FALSE as well as TRUE for sql.immediate
- added stop when attempting to replace (sql.replace == T) when not executing immediately (sql.immedate == F) 
- removed all-whitespaced lines as well as I could 

To be discussed: implement non-passing of parameter immediate when options$sql.immediate is not set.

Fix regex for lifting temporary table name to replace when sql.replace =T + a correction to the getQuery call with params where  `sql` was used instead of `query`.