Enable interrupt generation

[guiescuissato]

Hello! The motivation for me creating this pull request is being able to generate interrupts on a GY-521 board. I basically created 2 functions, to enable and disable interrupt generation:

void enableInterrupt(mpu6050_int_source_t int_source);
void disableInterrupt(mpu6050_int_source_t int_source);

The enum 'mpu6050_int_source_t' specifies which type of interrupt generation source should generate an interrupt, according to register 0x38:

• FIFO Buffer Overflow Interrupt
• I2C Master Interrupt
• Data Ready Interrupt

Since I only required the Data Ready Interrupt for my project, I didn't get to test the FIFO Buffer Overflow Interrupt or the I2C Master Interrupt. But, as I believe you'll agree, their enabling should work well with these functions.

To show you how I intended to use these functions, I also created an example that worked on an Arduino Pro Micro with the GY-521 board. The name of the example is 'interrupt_readings'. To better organize these changes, I did 2 commits, one for editing the library and another for creating the example. (edit: a 3rd to apply clang-format)

I hope you find these changes relevant and valuable! They certainly will be valuable to me, at least ;)

[Nurgak]

Can the conflicts be fixed and this PR merged? It contains very useful functionality that is currently missing form the library, notably: trigger the interrupt pin when updated values are available.