export command - help does not contain all options

[0xc0170]

I noticed that using mbed export --help does not list all the options, like compile does for instance.

mbed export --help
usage: mbed export [-h] -i IDE [-m MCU] [-v] [-vv]

Generate IDE project files for the current program.

optional arguments:
  -h, --help           show this help message and exit
  -i IDE, --ide IDE    IDE to create project files for. Example:
                       UVISION,DS5,IAR
  -m MCU, --mcu MCU    Export for target MCU. Example: K64F, NUCLEO_F401RE,
                       NRF51822...
  -v, --verbose        Verbose diagnostic output
  -vv, --very_verbose  Very verbose diagnostic output

There's --source feature I knew about for instance

[screamerbg]

Implemented in the latest PR. Please let me know if that's not the case.