Added RN props option, textInputProps to pass into text input.

[dblock]

Building on top of #49, but with a bit of README. This is very useful for all kinds of available text input properties, similar to switch properties. Some may very well be exposed on the parent component, but this is at least future proof and works for secureEntry or autoCapitalize right now.

                    <SettingsList.Item 
                      id='password' 
                      title='password' 
                      isEditable={true} 
                      hasNavArrow={false}
                      titleStyle={styles.settingsItemTitle}
                      textInputProps={{
                        autoCapitalize: 'none',
                        secureTextEntry: true
                      }}
                      onTextChange={(text) => this.setState({ 
                        user: { 
                          ...this.state.user,
                          password: text
                        },
                        dirty: true
                      })}
                    />

[dblock]

Anyone wants to merge this?