add inlineseq_(split,add) tools

This PR adds the following tools:
- `gt inlineseq_split` to split annotations and inline sequences into different files (or one of them to stdout, depending on which output file is given), and
- `gt inlineseq_add` to add sequences for all seqids in a given annotation, Sequences are taken from a sequence file (FASTA,EMBL,GenBank,...) and added to a GFF3 file as inline sequences.

Closes #342.
Currently only GFF3. You are right, if the format is mentioned in the `-gff3file` parameter it should be mentioned in the input as well.