T-Rex 0.19.9

* Add ability to execute custom scripts upon certain events with new parameters:
"--script-start"
"--script-epoch-change"
"--script-crash"
"--script-low-hash"
* ("ethash", "kawpow", "octopus") Add "--dag-build-mode" parameter to fine tune DAG build mode (mostly to help with various stability issues, see readme for details)
* Allow non-existent GPU indices passed in to "--devices" / "-d" parameter: the miner will display a warning message but will continue mining using the GPUs it could find)
* Cosmetic changes: improve error descriptions, display block number when mining solo at some pools (2miners)

Bug fixes:
* ("ethash", "kawpow", "octopus") Stability regression for Pascal GPUs
* Mining pool difficulty is not always updated in the summary report (https://github.com/trexminer/T-Rex/issues/131)

**NOTE**: if you experience stability issues when mining "ethash" right at the start or upon epoch change, e.g. some GPUs fail start with "GPU is idle" or "NVML: can't get GPU#" errors, try running with "--gpu-init-mode 1", it will force sequential GPU initialisation and reduce the load on your hardware (set to 2, 3, etc to indicate how many cards may be generating DAG in parallel)