* Implement an option to configure the echoed message, #48
Via `g:ale_echo_msg_format` where:
- `%s` is the error message itself
- `%linter%` is the linter name
- `%severity` is the severity type
e.g
let g:ale_echo_msg_fomat = '[%linter%] [%severity%] %s'
* Add new options for defining the string used for errors in echoed
message
`g:ale_echo_msg_error_str` and `g:ale_echo_msg_warning_str`
* Change text output of some linters
Now that the echoed message can be customized, no need to add the type
to the text variable.
* Update README & documentation file
* Fix some typos
* Sort the table of options alphabetically (except echo_msg_x_str options)
* Added echo warning str option to the doc
