ncbi_children error fixes and enhancement

I fixed a couple of errors I noticed when using `ncbi_children`:
1) Spaces in a searched name (e.g. a binomial species name) caused an error. I replaced them with `+` in accordance with the NCBI API.
2) Being unable to find a taxon id caused an error. Now, it returns NA if that happens. 

I also added an option called `ambiguous` that removes taxa with words like "unclassified", "unknown", "uncultured", or "sp." in their name. They are removed by default. Feel free to change the default value if you think ambiguous taxa should be included by default.