New feature request - support customs indicators and strategies

**New feature**
Look, it's not a problem but a possibility to use custom indicators.

I'm using FinRL almost three months and now a days I'm trying to understand the FinRL core functionalities to find the possibility of where I can contribute. Last couple of days ago I have created a financial strategy based on the "x" indicators which is not yet available in FinRL library.

**Solution**
1. Create an Object which will responsible for custom (indicator/strategy)
2. Finally can call the new object of "x" to the environment.

'''Like'''

FeatureEngineer(use_technical_indicator=True, tech_indicator_list=CUSTOM_INDICATORS,
use_turbulence=True,
user_defined_feature=False)

**Alternatives**
To backtesting my strategy, I have to use different environments (Backtesting) but it's not support RL yet, so for this reason I have to create a RL own environment.

**Additional context**
I believe, there are a lot of users who want to use this type of functionality.

after reading the code I found how to implement it. If you're agreeing with me to add this feature then I would love to work for it.