Report not allowed characters in identifiers

[zdne]

Currently only some characters are allowed in API Blueprint identifiers.

If illegal character is used the whole section is silently ignored.

Following blueprint should at least produce a warning about illegal identifier (character ')

# Group Parcel 
## Parcel's sticker [/parcels/{parcel_id}/stickers.json]

[zdne]

Also related: #70

[zdne]

With #70 resolved this only mean that unexpected characters in identifiers are [, ], ( and ).

E.g:

Group Parc ] el

[pksunkara]

What is needed to be done for this to be closed?

[zdne]

Currently a group like this is silently ignored (and treated as a description):

## Group ]A
Hello

To fix this issue the parser should warn that while keyword was find the identifier is not valid. Related issues are #31 and #65 and should be probably solved in one go.

[abtris]

Migrated to apiaryio/drafter#650