JPEG support for ionic cordova resources

**Description:**
ionic cordova resources cannot handle jpg files

**Steps to Reproduce:**
create a splash.jpg in resources/ and remove splash.png

**Output:**
fhapp-client shokrireza$ ionic cordova resources --verbose
[DEBUG] CLI flags: { interactive: true, confirm: false }
[DEBUG] { cwd: '/Users/shokrireza/git/fhapp-client', local: true, binPath:
'/usr/local/lib/node_modules/ionic/bin/ionic', libPath:
'/Users/shokrireza/git/fhapp-client/node_modules/ionic/dist/index.js' }
[DEBUG] Daemon found (pid: 81009)
[DEBUG] resourceJsonStructure=3
[DEBUG] platformEngines=android, ios
[DEBUG] buildPlatforms=android, ios
[DEBUG] getProjectPlatforms completed - length=2
[DEBUG] imgResources=50
[DEBUG] createImgDestinationDirectories completed - length=4
[DEBUG] getSourceImages completed - 1
[ERROR] Source image files were not found for the following platforms/types: android/splash, ios/splash

**My `ionic info`:**

cli packages:

@ionic/cli-utils : 1.12.0
ionic (Ionic CLI) : 3.12.0

global packages:

cordova (Cordova CLI) : 7.0.1

local packages:

@ionic/app-scripts : 3.0.0
Cordova Platforms : android 6.2.3 ios 4.4.0
Ionic Framework : ionic-angular 3.7.0

System:

ios-deploy : 1.9.1
ios-sim : 6.0.0
Node : v6.11.3
npm : 5.4.2
OS : macOS Sierra
Xcode : Xcode 9.0 Build version 9A235

Misc:

backend : legacy

Comece com o comando `ionic cordova resources` e as etapas de descoberta de recursos de imagem mostradas na saída, especialmente `getSourceImages`. Reproduza a falha com um `resources/splash.jpg` e sem `splash.png`; a tarefa estará concluída quando o comando reconhecer o JPEG tanto para os recursos de splash do Android quanto do iOS.

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