Allow for customizing the size of generated comment IDs

[hispanic]

This addresses eduardoboucas#401 - "Allow for customizing the size of generated comment IDs"

Two new config options have been added to the JSON-based config:

commentIdGenerator: {
  doc: 'The scheme to use for generating IDs for comments. More info about nanoid - https://github.com/ai/nanoid',
  format: ['uuid', 'nanoid', 'nanoid-lowercase'],
  default: 'uuid',
  env: 'COMMENT_ID_GENERATOR'
}

commentIdLength: {
  doc: 'The desired length of generated comment IDs. Only applicable when commentIdGenerator is set to "nanoid" or "nanoid-lowercase"',
  format: Number,
  default: 21,
  env: 'COMMENT_ID_LENGTH'
}

Currently, Staticman creates all comment IDs as UUIDs. While using UUIDs is a fool-proof solution that basically guarantees the uniqueness of IDs, it can be overkill. Consider a small blog that receives, at most, one comment each day/week/month/etc. For many/most users, the nature of the IDs that Staticman generates will not matter. However, the Staticman configuration allows for the generated ID to be exposed, when desired. For example, it can be incorporated into the filenames generated by Staticman.

For usage scenarios such as these, it is desirable to be able to configure Staticman to generate shorter/simpler comment IDs. Nano ID has been added to achieve this.

For example, if commentIdGenerator is set to nanoid-lowercase and commentIdLength is set to 8, IDs akin to the following will be generated: ry6-bt9d

According the the Nano ID Collision Calculator, for a blog receiving one comment submission every hour, "~34 years [would be] needed, in order to have a 1% probability of at least one collision".