修改textFieldplaceholder字体颜色和大小

<code class="bash">textField.placeholder = @<span class="hljs-string" style="color: rgb(0, 136, 0);">"Username!"</span>;
[textField <span class="hljs-built_in" style="color: rgb(102, 0, 102);">set</span>Value:[UIColor redColor] <span class="hljs-keyword" style="color: rgb(0, 0, 136);">for</span>KeyPath:@<span class="hljs-string" style="color: rgb(0, 136, 0);">"_placeholderLabel.textColor"</span>];
[textField <span class="hljs-built_in" style="color: rgb(102, 0, 102);">set</span>Value:[UIFont boldSystemFontOfSize:<span class="hljs-number" style="color: rgb(0, 102, 102);">16</span>] <span class="hljs-keyword" style="color: rgb(0, 0, 136);">for</span>KeyPath:@<span class="hljs-string" style="color: rgb(0, 136, 0);">"_placeholderLabel.font"</span>];</code>