Resolve "Personalised Responses (comments)"

This implements support for placeholders in a simple way.

Supported placeholders are:

• created_at: the resource's creation date
• updated_at: the resource's last update date
• closed_at: the resource's closed date (if applicable)
• merged_at: the resource's merged date (if applicable)
• state: the resources's current state: opened, closed, merged
• author: the username of the resource's author as @user1
• assignee: the username of the resource's assignee as @user1
• assignees: the usernames of the resource's assignees as @user1, @user2
• closed_by: the user that closed the resource as @user1 (if applicable)
• merged_by: the user that merged the resource as @user1 (if applicable)
• milestone: the resource's current milestone
• labels: the resource's labels as ~label1, ~label2
• upvotes: the resources's upvotes count
• downvotes: the resources's downvotes count

Use placeholders with double curly braces, e.g. if the author's username is rymai, then Hey {{author}}, please have a look! would be expanded to Hey @rymai, please have a look!.

Fields with multiple values are joined with , . For instance Labels for this issue are {{labels}}. would be expanded to Labels for this issue are ~"In dev", ~"Quality", ~"feature proposal".

Fields unsupported by the resources or that are nil are not replaced.

Closes #29 (closed)