Fix -startpos flag being ignored (#1129)

Fix -startpos flag being ignored (#1129)

* Refactored cursor location login into a function. Fixed buffer overflow when line position is 1 more than file lines

* Fixed crash when -startpos has an invalid argument

* Adapted tests to new interface

* Fixed bug where -startpos with lines 0 and 1 would both be on the first line

* Changed Fatalf format back to digits

* Fixed issues with buffer cursor location. Added tests for new function

* ParseCursorLocation will now return an error when path doesnt contain line/col

* Fixed off-by-one line error

* Fixed tests to account for subtracting 1 from the line index