Add an option to override/complement regex for JIRA specification check

[iamfeniak]

When using JIRA format for specification check, e.g.

specification:
    allowed_format: ['jira']

it would be useful to have a parameter to override/complement default regular expression used for validation.

Example current behaviour :

specification:
    allowed_format: ['jira']

"FIRST-42 Feature development" is valid commit message.
"SECOND-42 Feature development" is valid commit message.

Example desired behaviour :

specification:
    allowed_format: ['jira']
    allowed_schemes: ['FIRST-*']

"FIRST-42 Feature development" is valid commit message.
"SECOND-42 Feature development" is invalid commit message.