Methylartist

Various minor fixes and added options:
- [locus/region: --smoothalpha option](https://github.com/adamewing/methylartist/commit/8b86ffa921db2110c70a1e0cc988b2b9ec97f9da)
- [region: --color_by_hp](https://github.com/adamewing/methylartist/commit/78ee40edc090b54c3a0e99af60eda4565f405a0a)
- [locus: more masking options (`--maxmaskedfrac` and `--nomask`)](https://github.com/adamewing/methylartist/commit/681522f3183f3d9538dada7908bbfe27cd2ef411)

* `segplot:`better control of group by sample versus group by annotation (`-a/--group_by_annotation`)
* `scoredist:` bugfixes and appearance parameters (`--palette` and `--lw`)
* `locus:` option to colour by haplotype (`--color_by_hp`), option to not include 'PS' tag in haplotype ID (`--ignore_ps`)
* `region:` option to control gene density / stacking (`--gene_track_height`)
* minor tweaks and bugfixes

Quite a few new features:

- Support for .bam files with modified base tags (`Mm`/`Ml`) (no .db required if using mod bams) via `-b`/`--bam`
- Support for conversion of arbitrary per-base per-read methylation table data into .db (`db-custom`)
- Ridge plots in segplot (`-g`/`--ridge`)
- Add parameters to modify the read mapping panel in locus plot (`--readmarker`, `--readmarkersize`, `--readlinewidth`)

- bugfixes
- support `--phased` in `region` plots

* add `--spanning_only` flag to `segplot`
* add parameter info to `segplot` output filename
* bugfixes

* add `--spanning_only` option to segmeth
* minor bugfixes