Custom Format and Placeholders

[alfonsusac]

Subject of the feature

Add the ability to customize format of every text in the embed. Which meant that placeholders has to be added too.
Placeholders are used for formats such as:

• {date} = calendar's date (according to localisation)
• {dayofweek} = day of the week (according to localisatin)
• {start} = event start time (according to localisation)
• {end} = event end time (according to localisation)
• {title} = event title
• {desc} = event description
• {loc} = event location

For example, the current default format would be:

header = *{dayofweek}* - {date}
event = ```{start} - {end} | {title}```
which can be customised to however the user wants the calendar to be.

The user can access the following command to customise the embed:
!displayoption formatheader <string>
!displayoption formatevent <string>

For example, if i would like the title and desc to be displayed but separated on a new line, the command would be
!displayoption formatevent ```{start} - {end} | {title}\n{desc}```

Problem

Although the customization of this bot is already superb, customization still feels too limiting. Customization can be more dynamic with the addition of formatting and placeholders.

EDIT: fixing formatting

[mchangrh]

this is cool but I'm going to save this for the slash command rewrite otherwise validation will be absolute hell