Release of v1.1.0

Interface changes:

• 'read_vcfs_as_granges': The 'genome' parameter
  must now be the name of a BSgenome library, to prevent
  problems with seqlevels style.

  The function now accepts an optional 'group' parameter to
  use a subset of chromosomes.

  It also accepts the new optional 'check_alleles'
  parameter to significantly speed up the reading of VCF files.

• 'plot_contribution': This function now accepts an
  optional parameter 'palette' to specify custom colors.

Performance updates:

• Implement parallel execution in 'read_vcfs_as_granges',
  'mut_matrix' and 'mut_matrix_stranded'.

Bugfixes:

• Fix 'mut_type_occurences' to handle missing types.
• Fix 'mut_matrix' and 'mut_matrix_stranded' to emit warnings
  when processing empty GRanges.
• Fix inconsistencies in the README and the vignette.

Other changes:

• Various vignette updates.
• Added unit tests for 'read_vcfs_as_granges', 'mut_matrix',
  and 'mut_matrix_stranded'.