fix: filenames with spaces passed to command line

see more details @ http://stackoverflow.com/questions/17897359/error-with-tex2docx-function-from-reports-r-package

Demo:

```r
library(reports)
dir.create('foo bar')
DOC <- system.file("extdata/doc_library/apa6.qual_tex/doc.tex", package = "reports")
file.copy(system.file("extdata/docs/example.bib", package = "reports"), 'foo bar/example.bib')
BIB <- 'foo bar/example.bib'
tex2docx(DOC, file.path(getwd(), "test2.docx"), path = NULL, bib.loc = BIB)
```