tag v0.2.0

Added

• An index class for efficient in-memory access of sequence features.
• Module for mRNA handling, with a function for selecting the primary mRNA from
  a gene or other feature.
• New CLI command tag pmrna.
• A new Score class for internal handling of feature scores. Not yet included
  in the API, and may not ever be.

Changed

• Modules focused on classes / data structure now support more concise imports
  (for example, from tag import Feature and tag.Feature now supported and
  preferred over from tag.feature import feature and tag.feature.Feature).

Fixed

• Resolved a bug with the GFF3Writer failing to print ##FASTA directives
  before writing sequences to output.