Add feature to support navigating to a specific column and line, as well as specific line

Fixes #16.

Allows an entry in the go-to-line box containing a colon, which acts as a separator between the line number and column such that:
  - If a line number and a column number are specified, go-to-line will:
    - Navigate to the specified line and column, if both the line number and column number are within boundaries
    - Navigate to the specified line and the last column in the line, if the column number is greater than the number of columns in the specified line
    - Navigate to the specified line and the first column in the specified line, if the column number is less than or equal to 1.
    - Navigate to the last line and last column in the file if the line and column are both out of bounds
 
![](http://i.gyazo.com/5a79c6af4cc8f2d9692a31fa882b4158.gif)
  - If a line number was specified only, go-to-line will retain its current functionality.
  - If a colon is specified, followed by a column number, go-to-line will:
    - Navigate to the specified column within the line the cursor is currently located.

Also added in this commit:

- :white_check_mark: Add a test for behavior when line entered exceeds number of lines in the buffer.
Do you need the `\` before the `:`? I didn't think that character needs escaping.