Make third parameter of ifx optional

[kabirbaidhya]

• Make the third parameter for {{ifx}} optional with a default value empty string ("").
• Closes Improvements on ifx helper #34

So, finally we can write this

<li class="{{ifx isActive 'active'}}">
<li class="{{ifx isUnavailable 'unavailable'}}">

instead of

<li class="{{ifx isActive 'active' ''}}">
<li class="{{ifx isUnavailable 'unavailable' ''}}">
<li class="{{ifx isUnavailable 'unavailable' ''}}">

I hope it's more readable and makes better sense.

[codecov-io]

Current coverage is 98.47%

Merging #35 into master will increase coverage by +<.01%

@@             master        #35   diff @@
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  Files             7          7          
  Lines           129        131     +2   
  Methods           0          0          
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+ Hits            127        129     +2   
  Misses            2          2          
  Partials          0          0          

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