Add more help to errors when linting

```
> make lint go run tools/lint/lint.go service-manual error parsing Markdown in
service-manual/agile/features-of-agile.md: Error at line 32, column 7 (file
offset 898):
  31:
  32: Each [
           ^
```
Handle UTF-8 input which is a variable width-encoding scheme. ’ is an example of a character that needs more than a single byte to encode it, but which will display as a character which only takes up one column.