Elm language resources added (#3054)

* added Solidity

* added Solidity

* added Solidity

* bitcoin book added

* fixed ordering on bitcoin book entry

* added authors for bitcoin and solidity books

* added Bitcoin and cryptocurrency Princeton book

* elm interactive playground added (ellie)

* free elm books added

* elm podcast added

* corrected spacing

* reordered elm books into existing category

* reordered format and author

* removed trailing /s

* fixed alphabetical ordering

free-podcasts-screencasts-en.md

@@ -16,6 +16,7 @@
 * [Haskell](#haskell)
 * [Javascript](#javascript)
   * [Angular.js](#angularjs)
+  * [Elm](#elm)
   * [Ember.js](#emberjs)
   * [Node.js](#nodejs)
   * [React.js](#reactjs)
@@ -143,6 +144,11 @@
 * [Angular Air](https://angularair.com) (podcast)
 
 
+#### Elm
+
+* [Elm Town](https://elmtown.audio) (podcast)
+
+
 #### Ember.js
 
 * [Ember Weekend](https://emberweekend.com/episodes) (podcast)