Allow use of pandoc filters

[lggruspe]

Is your feature request related to a problem? Please describe. There's currently no way to use slipbox with custom pandoc filters.

Describe the solution you'd like

Allow passing filters via CLI args or config file:

slipbox build --filters './filters/foo.lua:./filters/bar:./filters/baz.lua'

Or with config file:

[slipbox]
# omitted ...

[pandoc-options]
filters = filters/foo.lua:filters/bar:filters/baz.lua

# or:

[pandoc-filters]
filters/foo.lua = true
filters/bar = true
filters/baz.lua = true