use 'fixed = TRUE' where possible

[ChrisMuir]

I was poking around in this repo some, figured I'd submit this. The majority of this PR just adds arg fixed = TRUE wherever possible, which is an easy way to get a small speed boost from functions strsplit(), grepl(), and gsub().

The only other edit is to add arg stringsAsFactors = TRUE to the call to as.data.frame() in line 179 of KWIC.R (see here ). Without this arg, variable Var1 from table() ends up as a factor, and when you subset kwf in line 180, that variable holds onto all of the levels that existed prior to subsetting, which makes the final data frame kwf a much larger object than it needs to be.

Let me know if you want anything tweaked, I'm happy to make edits if need be.