Allow selecting time format

[Testato]

Today i needed formatted result only on HHmm, so i modified the get formatted metodhs.
So if you agree it is a good idea create a new constructor whit a new argument passed for select some formatted result:

• HHmm
• HHmmss
• Etc.

[FWeinb]

I don't think adding time formatting to this library is a good idea. In my opinion any further time formatting should be done externally. The getFormattedTime() function is just to get you started.

[sandeepmistry]

I agree with @FWeinb on this.

If it doesn't exist already, a separate date formatting/parsing Arduino library would be ideal.

[Bif1005]

According to me I am missing: getMonth, getYear and getDay (not
day of the week)

[blackketter]

For general clock functions, I use the excellent https://github.com/PaulStoffregen/Time library and use NTP as a time sync provider.

#include "TimeLib.h"
time_t ntpSyncProvider() {
  return timeClient.getEpochTime();
}

then:

  setSyncProvider(&ntpSyncProvider);

That library does include an NTP example but I prefer using this NTPClient.