Bugfix: Issue #4 - Error saving a new file more than once

• Removed fields having to do with file name and path from the global state in the main package.
• Reverted NewScreenBuffer to just take a file pointer, because the name of the a file already contains the path.
• Added path/filepath import to screenbuf.go to work with paths on NewScreenBuffer().
• Fixed bug on pressing enter when the buffer length is greater than the default height. Bug was introduced when trying to scroll text when adding lines to the bottom.
• Added row (cursorPos.y) to AddLineToBuffer(int,int) parameter list.
• Re-added code to deal with loading a file, reading a line with no \n, EOF at the end and there's still data read in the buffer.