fix(input): add variables for alert and input placeholder color

closes #10153

Author: brandyscarney

src/components/alert/alert.scss

@@ -15,6 +15,9 @@ $alert-button-line-height:        20px !default;
 /// @prop - Font size of the alert button
 $alert-button-font-size:          14px !default;
 
+/// @prop - Color of the alert input placeholder
+$alert-input-placeholder-color:   #999 !default;
+
 
 ion-alert {
   position: absolute;
@@ -75,7 +78,7 @@ ion-alert input {
 }
 
 .alert-input {
-  @include placeholder();
+  @include placeholder($alert-input-placeholder-color);
 
   padding: 10px 0;
 

src/components/input/input.scss

@@ -9,6 +9,9 @@ $text-input-highlight-color-valid:       #32db64 !default;
 /// @prop - Color of the input highlight when invalid
 $text-input-highlight-color-invalid:     #f53d3d !default;
 
+/// @prop - Color of the input placeholder
+$text-input-placeholder-color:           #999 !default;
+
 
 // Input/Textarea Wrapper
 // --------------------------------------------------
@@ -41,7 +44,7 @@ ion-textarea {
 // --------------------------------------------------
 
 .text-input {
-  @include placeholder();
+  @include placeholder($text-input-placeholder-color);
   @include appearance(none);
 
   display: inline-block;