Echo string format (#76)

* 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

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