Add mermaid ER diagram

[maurerle]

Using -m mermaid_er one can use the Mermaid ER diagrams.

This functionality is added in this PR similar to the existing mermaid class diagram. Unfortunately, the mermaid diagrams are not compatible with special characters, colons and spaces in a lot of parts: https://github.com/mermaid-js/mermaid/issues/1546

Therefore, colons and spaces are replaced with an underscore to increase compatibility and create nice diagrams:

The not null check is currently not supported as well as setting FK for ForeignKeys, as this information is not available in the Column class yet.

Old class diagram

classDiagram
class post_tags{
 *INTEGER post_id NOT NULL
   *INTEGER tag_id NOT NULL
}
class users{
 *INTEGER id NOT NULL
   VARCHAR<80> activation_key
   DATETIME date_joined
   VARCHAR<150> email NOT NULL
   BOOLEAN email_alerts
   TEXT followers
   TEXT following
   INTEGER karma
   VARCHAR<80> openid
   VARCHAR<80> password
   BOOLEAN receive_email
   INTEGER role
   VARCHAR<60> username NOT NULL
}
class posts{
 *INTEGER id NOT NULL
   INTEGER access
   INTEGER author_id NOT NULL
   DATETIME date_created
   TEXT description
   VARCHAR<250> link
   INTEGER num_comments
   INTEGER score
   TEXT tags
   VARCHAR<200> title
   TEXT votes
}
class comments{
 *INTEGER id NOT NULL
   INTEGER author_id NOT NULL
   TEXT comment
   DATETIME date_created
   INTEGER parent_id
   INTEGER post_id NOT NULL
   INTEGER score
   TEXT votes
}
class tags{
 *INTEGER id NOT NULL
   VARCHAR<80> name
   VARCHAR<80> slug
}
posts "1" -- "0..n" post_tags
tags "1" -- "0..n" post_tags
users "1" -- "0..n" posts
users "1" -- "0..n" comments
posts "1" -- "0..n" comments
comments "0..1" -- "0..n" comments

New ER Diagram

erDiagram
post_tags {
 INTEGER post_id PK
   INTEGER tag_id PK
}
users {
 INTEGER id PK
   VARCHAR(80) activation_key 
   DATETIME date_joined 
   VARCHAR(150) email 
   BOOLEAN email_alerts 
   TEXT followers 
   TEXT following 
   INTEGER karma 
   VARCHAR(80) openid 
   VARCHAR(80) password 
   BOOLEAN receive_email 
   INTEGER role 
   VARCHAR(60) username 
}
posts {
 INTEGER id PK
   INTEGER access 
   INTEGER author_id 
   DATETIME date_created 
   TEXT description 
   VARCHAR(250) link 
   INTEGER num_comments 
   INTEGER score 
   TEXT tags 
   VARCHAR(200) title 
   TEXT votes 
}
comments {
 INTEGER id PK
   INTEGER author_id 
   TEXT comment 
   DATETIME date_created 
   INTEGER parent_id 
   INTEGER post_id 
   INTEGER score 
   TEXT votes 
}
tags {
 INTEGER id PK
   VARCHAR(80) name 
   VARCHAR(80) slug 
}
posts 1--0+ post_tags : has
tags 1--0+ post_tags : has
users 1--0+ posts : has
users 1--0+ comments : has
posts 1--0+ comments : has
comments one or zero--0+ comments : has

closes #6