Add optional CLI parameter for organism to main app

[uniqueg]

In order to select the transcripts that the reads should be mapped to in later steps (see #26 and #27) of the read orientation inference process, the organism that the sample was derived from needs to be known. As we currently do not have a functionality to infer the organism from the data (it has been started and is almost ready, but has not been merged), we need to be able to pass that information when we are executing the read orientation inference (#23).

Add a command-line interface (CLI) parameter --organism that either takes an organism short name (a string) or a taxon ID (an integer). Have a look at data/transcripts.fasta to see how these look like (4th and 5th fields of identifier lines, respectively). Ensure that passing --organism when calling htsinfer.py is optional. Validate that the input is either of type str or of type int. Don't forget to document the new CLI parameter in the repo's README.md.

[uniqueg]

When you set an optional CLI parameter, you will need to set some default (in case it is not provided). For now, a reasonable default could be hsapiens. Later on, once the organism inference functionality is in, we would probably set it to None so that it would be inferred by that functionality by default, and that inference would be skipped if the user provides an organism explicitly.