custom track widgets feature update + docs fixes

[sachin-dahal]

Thankful for the wonderful package!

This PR consists of making custom track widgets available. Using custom widgets instead of activeText and inactiveText was creating a problem for me and others as well. This PR solves the issue and makes the package users available to use custom widgets in place of activeText and inactiveText. activeText and inactiveText can also be used but if activeTrackWidget or inactiveTrackWidget are enabled, activeText and inactiveText cannot be used correspondingly along with their properties.

All tests are also been cleared

Closes issue #40

also, few witnessed issues have been fixed inside the main documentation page along with the updates of features of this PR

Thank You!

[mitesh77]

I think @sachin-dahal you have to create a new FlutterSwitch Library, This one has no response.