Change style of trigger component

[Chng-Zhi-Xuan]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [X] Bug fix

Fixes MarkBind/markbind#111

What is the rationale for this request?

• User encountered bug where highlighted text blocks underlines for trigger elements.

What changes did you make? (Give an overview)

• Modified trigger.vue at trigger.style to use ['text-decoration'] instead of ['border'].

Provide some example of changes:

Before	After

Note:
Triggers have a different style when set to "click", which will use dashes. 
Otherwise it defaults to dotted. However, popovers and tool-tips can also 
set "tigger" option to "click" but their style remains as dotted.

Might want to ensure consistency between components in another issue / PR

Is there anything you'd like reviewers to focus on?

• Code style

Testing instructions:

1. Type highlighted text using either <mark>text</mark> or <md>==text==</md> below a trigger element as shown in the example above.