add support for Honeywell HPMA115S0 particle sensor

[climberhunt]

Description:

The Honeywell HPM Series Particulate Matter Sensor is a
laser-based sensor which detects and counts particles using
light scattering.

This commit adds:
  * The HPMA115S0 library by Felix Galindo (seems lightweight enough for Tasmota)
  * selectable GPIOs for Tx and Rx (SoftwareSerial only)
  * xsns_56_hpma.ino for Tasmota sensor implementation

Signed-off-by: David Hunt <dave@davidhunt.ie>

Checklist:

• The pull request is done against the latest dev branch
• Only relevant files were touched
• Only one feature/fix was added per PR.
• The code change is tested and works on core pre-2.6
• The code change pass travis tests. Your PR cannot be merged unless tests pass
• I accept the CLA.

[climberhunt]

Travis build is failing. I probably need to go change the use of SoftwareSerial. I didn't spot this as it builds and functions fine on my setup.
Any comments on the patchset? Would it make a reasonable addition?

[climberhunt]

SoftwareSerial error fixed by switching over to TasmotaSerial.
However, now failing because I've a new library added in the patch for the HPMA115 device.
The 'case' of the file is in uppercase, whereas ino looks for lower case. Will push update.