Allow "no sorted" Icons & add class for sorted columns & allows to cancel unfinished request

[alfonsobries]

There are three useful changes, hope you can join them to the next version or at least use as an idea:

1. Allow "no sorted" Icons
   Allows to optional add a "no sorted" icon, im not sure what icon library u use but in my case i add a double (up and down) chevron icon when the column is not sorted so its more clear for the user that they can sort by clicking the title

2. Add class for sorted columns
   Useful to add some kind of class for sorted columns so you can add styles for "sorted" columns and make more clear that the content is ordered by this columns

3. Cancel unfinished request when new request was sended (My favorite, please add this)
   If the user quickly order a row many times, and have an unfinished request sometimes multiple "loadData" request was sended, this use much memory and in some cases the oldest request finish last so the current data is innacurate, this is more problematic with custom filters like a search input where the user type and the data was live refreshed. With this changes if the loadData funcion has a previous unfinished request it cancel it so you only have one request at a time.

PD: Sorry for the "testing" commit

[ratiw]

@alfonsobries Thanks for the PR. It's really appreciated.

1. The "no sorted" icon is actually been submitted in this PR Add default icon to css #2 #192 as defaultSortIcon and has been merged to the develop branch as sortableIcon.

2. Class for sorted columns is not need as you can use sortable class to style it the way you added sorted class, so it's redundant.

3. The cancel request is ok, but please make a separate PR based on the develop branch and re-submit again.